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If vs. Unless

Express specific conditions for an outcome

If vs. Unless

IF
A conditional sentence with if expresses that "an action or situation will happen after the other one happens first." The action in the clause following if: (1) expresses a condition that refers to a particular situation; (2) expresses general timing if it is for multiple situations "whenever".
2ND ACTION	1ST ACTION
DESIRED OUTCOME	A PARTICULAR CONDITION
You will have a tender turkey	if you cook it slowly. (a singular situation)
You will have a tough turkey	if you overcook it.
We'll arrive at 8:00	if our train is on time.
We'll bring some champagne	if you wish.
DESIRED OUTCOME	A GENERAL SITUATION
(+) You will always have a tender turkey	if / when / anytime / whenever you cook it slowly. (+) (on multiple occasions; routine response; expresses timing or frequency)

UNLESS "EXCEPT IF"
A conditional sentence with unless expresses "an action or situation will happen if the other one does not happen first" or "an action or situation won't happen if the other one happens first." (See polarity¹.) Unless expresses "except if" or "if not". +positive /–negative wording
2ND ACTION	1ST ACTION
+ / – OUTCOME	+ / – PARTICULAR CONDITION
(+) You will have a tender turkey (–) You won't have a tender turkey	unless you overcook it. (–) unless you cook right. (+)
You will have a tough turkey	unless you cook it slowly.
We'll arrive at 8:00	unless our train is late.
We'll bring some champagne	unless you object.
DESIRED OUTCOME	+ / – GENERAL SITUATION
(+) You will have a tender turkey You won't have a tender turkey	unless you overcook it. (–) if / when / anytime / whenever you overcook it. (on multiple occasions; routine response; expresses timing or frequency)

adverse (Adj) – not favorable, in opposition (bad)

if (P) – expresses the requirement for a particular situation to occur. He will cry if you leave. (condition)

if (P) – expresses the happening of a situation repeatedly. He cries if / when / whenever you leave. (timing) See if vs. when.

overcook (V) – cook too much, past a desired state (a negative action).

¹polarity – words that are sensitive to the positivity or negativity of the surrounding words. (if/unless, some/any, either, neither, already/yet, etc.) See Positive & Negative Polarity, Some / Any Polarity, Already / Yet Polarity, Too / Either Polarity.

recurring (Adj) – a repetitive or habitual situation that happens at regular intervals;

reoccuring (Adj) – a situation that happens again but not repeatedly.

if (P) — "if" heads a connective prepositional phrase, which takes a clause, an adjective, an adverb or a clause with an infinitive as its complement. (See Other IF Complements.) What we have traditionally called a "conditional clause" is actually a prepositional phrass. For example, the phrases are marked with brackets: (1) Call me if you are ready. (you are ready – clause)

(2) Call me if necessary. (necessary – adjective)

(3) I rarely, if ever, call them. (ever – adverb)

(4) If I am to help, you must cooperate with me. (I am to help – clause w/infinitive)

Also see Grammar Notes below for grammar terms and see Conditional Summary for practices.

If & Unless Clause Position

Punctuation

Emphasis Clause Order

EMPHASIS PLACEMENT
The if or unless phrase can be placed before the outcome clause for emphasis. A comma is placed after the if phrase.
COMMA
If you wish, we'll bring some champagne.
Unless you object, we'll bring some champagne.

STANDARD PLACEMENT
When the if or unless phrase is placed after the main clause, no particular emphasis is intended, and a comma is not used.
NO COMMA
We'll bring some champagne If you wish.
We'll bring some champagne unless you object.

If and Unless

Tense Agreement

Tense Use

PRESENT / PAST
When discussing habits or routines, the present or the past tense can be used in conditional statements. See Real Conditions.
IF
We often arrive at 8:00 if our train is on time.
We often arrived at 8:00if our train was on time.(routinely)
UNLESS
We often arrive at 8:00 unless our train is late.
We often arrived at 8:00unless our train was late. (routinely)

FUTURE
However, when discussing future plans, the present tense is used for the future in the unless or if condition.
IF
We will arrive at 8:00 if our train is on time.
*We will arrive at 8:00 if our train will be on time. (use present)
UNLESS
We'll arrive at 8:00 unless our train is late.
*We'll arrive at 8:00 unless our train will be late. (use present)

*Yellow highlighting marks an example with incorrect usage.

Condition

Condition for success vs. likely failure

Condition for Success vs. Failed Outcome

IF / UNLESS
An if phrase poses a condition in order to achieve an outcome: if, only if, unless , provided that.
ONE WAY (OTHERS MAY EXIST)
If you use a thermometer, you will know when your turkey is done.
ONLY ONE WAY
Only if you use a thermometer, will you know when your turkey is done. (auxiliary verb goes before subject)
NOT DOING IT THIS WAY (– / +)
Unless you use a thermometer (+) you won't know when your turkey is done. (–)

OTHER EXPRESSIONS
In contrast, the outcome of an otherwise or an or else phrase states the likely outcome if you do not do the action in the clause or sentence before it.
ONE WAY
Use a thermometer. Otherwise, you won't know when your turkey is done. (Otherwise is followed by an independent clause)
ONE WAY
Use a thermometer, or else you won't know when your turkey is done.
SPECIFICALLY THIS WAY
Provided that you use a thermometer, you will have a delicious turkey. (+)

Only If

Word Order — Transposition

Beginning with Only if

CONDITION
Both an if and an only if phrase may be placed at the beginning of a clause. A comma separates the prepositional phrase with the condition from the main clause.
IF
If you dry your dishes with a towel,
If you use Zing dish soap,
If you used Zing dish soap,
ONLY IF
Only if you dry your dishes with a towel,
Only if you use Zing dish soap,
Only if you used Zing dish soap,

OUTCOME
Note that he outcome of the only-if phrase has the auxiliary verb placed before the subject.
STANDARD WORD ORDER
they will be spotless!
you get really clean dishes.
you would get really clean dishes. (conditional)
TRANSPOSED
will they [will] be spotless!
do you [do] get really clean dishes. (get = do get)
would you [will] get really clean dishes.

Commonly Confused

Only if vs. If only

A Condition vs. A Wish

ONLY IF
Only if expresses that one condition is required to achieve a desired effect. When used at the beginning of the sentence the auxiliary verb is moved before the subject. Only is a focusing adverb for if which is a preposition.
ONE CONDITION
Only if you dry your dishes with a towel, will they be spotless!
Only if you clean up your room, will you find your lost jeans.
Your windows will be clean enough to see your face only if you wash them with Zing!
I will please my mother-in-law only if my house is clean.
I am happy only if you are here. / Only if you are here, am I happy.
She was pleasant only if we told her what she wanted to hear.

IF ONLY
If only expresses a wish—one that you are doubtful about actually happening. The result clause is optional and is separate. It is more emphatic than I wish… If is a preposition and only is a focusing adverb for the content of the clause that follows.
I WISH…
If only I had more time! I could relax.
If only you would clean up your room. You would find your lost jeans.
If only I had some Zing window cleaner.
If only I could please my mother-in-law.
If only you were here!
If only he hadn't told his mother everything.

Also see Wishes (only if / if only)

If Only (Huddleston 751)

Commonly Confused

Even if / Whether or not

Even if & Whether or not

EVEN IF
Even if expresses that something is a challenging or negative condition. "No condition will change the outcome" Even if emphasizes the speakers intent to achieve the outcome or goal without regard for a difficult or negative condition. [Focus Adv + Prep]
OUTCOME	CHALLENGING CONDITION
I'll help you	even if I don't have much time.
I like to walk to work	even if it is raining.
Your father loves you	even if your father doesn't say it.
He'll get his work done	even if he has to work all night.

WHETHER OR NOT
Whether or not expresses that no condition (or its alternative) will stop the completion of the outcome in the main clause. That is, if the condition exists or if the condition does not exist, the outcome will or must happen anyway. Also called an exhaustive conditional.
OUTCOME	THE CONDITION DOESN'T MATTER
I'll help you	whether or not I have much time.
I like to walk to work	whether it is raining or not .
Your father loves you	whether or not he says it.
He'll get his work done	whether or not we wants to work all night.

Whether can be split apart from or not by the subject and predicate of the clause.
See If / Whether.

alternative (N) – something you can choose to do or use instead of something else
Also see even if Adverbs for Emphasis

(Huddleston 8 §14.1.3, 11 §5.3.6)

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Grammar Notes (Advanced)

Traditional and Linguistic Descriptions

TRADITIONAL and ESL GRAMMAR
if, unless If-clauses (also called "adverb clauses of condition") present possible conditions. The main clause expresses results. (Azar 17-6) Words that introduce adverb clauses of condition (if-clauses) if, whether or not, even if, in case, only if. true condition If you tell me about the problem, I will help. (True in the present or future) untrue condition If you had told me about the problem, I would have helped. (Information in the if clause is contrary to fact.) subordinating conjunctions introduce adverb clauses and signal the relationship between the adverb clause and another clause, usually an independent clause. (Lunsford 147) (Swan 257) Conditional sentences introduced by if, focus on questions of truth. Conditional sentences "make different assumptions about the likelihood that what is stated in the if clause is true, and then draws the corresponding conclusion in the main clause." (Lunsford 710)
Unless "if…not" means if…not (Azar 17-10) has a similar meaning to if…not , in the sense of 'except if". Come tomorrow, unless I phone. (Swan 601)
if, whether "When a yes/no question is changed to a noun clause, whether or if is used to introduce the clause. Whether is more common that if in formal English . Both whether and if are commonly used in speaking." (Azar 12-3) I don't know [whether he will play]. "noun clause" [Whether or not he will play] is unclear. (Azar 12-3)

TRADITIONAL and ESL GRAMMAR

if, unless

If-clauses (also called "adverb clauses of condition") present possible conditions. The main clause expresses results. (Azar 17-6)

Words that introduce adverb clauses of condition (if-clauses) if, whether or not, even if, in case, only if.

true condition If you tell me about the problem, I will help. (True in the present or future)

untrue condition If you had told me about the problem, I would have helped. (Information in the if clause is contrary to fact.)

subordinating conjunctions introduce adverb clauses and signal the relationship between the adverb clause and another clause, usually an independent clause. (Lunsford 147) (Swan 257)

Conditional sentences introduced by if, focus on questions of truth. Conditional sentences "make different assumptions about the likelihood that what is stated in the if clause is true, and then draws the corresponding conclusion in the main clause." (Lunsford 710)

Unless "if…not"

means if…not (Azar 17-10)

has a similar meaning to if…not , in the sense of 'except if". Come tomorrow, unless I phone. (Swan 601)

if, whether

"When a yes/no question is changed to a noun clause, whether or if is used to introduce the clause. Whether is more common that if in formal English . Both whether and if are commonly used in speaking." (Azar 12-3)

I don't know [whether he will play]. "noun clause"

[Whether or not he will play] is unclear. (Azar 12-3)

LINGUISTIC DESCRIPTION
if, unless preposition.conditional adjunct subordinator preposition heads an adjunct prepositional phrase that takes a clause as its complement if [ protasis (the proposition, the condition) + apadosis (the answer, consequence)] (Huddleston 8 §14) open conditional If he lies, everyone will know. If P (then) Q. remote conditional If he lied, everyone would know. P ≠ true. Q ≠ true. The remote construction differs from the open in that it entertains the condition as being satisfied in a world which is potentially different from the actual world." (8 §14.2.1)
Unless "except if" Unless occurs in open conditionals and less frequently in remote ones. (8 §14.3) We'll be there on time unless there is traffic. (open) I wouldn't say I would be there unless I could. (remote) Unless does not replace "if not". (Huddleston 756) We'll leave now unless you'd rather wait. "except if" *We'll leave now if not you'd rather wait.
whether vs. if subordinator takes a clause as its complement They are sometimes interchangeable but not always. (Huddleston 11 §5.2) See Subordinate Yes/No Questions.

LINGUISTIC DESCRIPTION

if, unless

preposition.conditional adjunct subordinator

preposition heads an adjunct prepositional phrase that takes a clause as its complement

if [ protasis (the proposition, the condition) + apadosis (the answer, consequence)] (Huddleston 8 §14)

open conditional If he lies, everyone will know. If P (then) Q.

remote conditional If he lied, everyone would know. P ≠ true. Q ≠ true.

The remote construction differs from the open in that it entertains the condition as being satisfied in a world which is potentially different from the actual world." (8 §14.2.1)

Unless "except if"

Unless occurs in open conditionals and less frequently in remote ones. (8 §14.3)

We'll be there on time unless there is traffic. (open)
I wouldn't say I would be there unless I could. (remote)

Unless does not replace "if not". (Huddleston 756)
We'll leave now unless you'd rather wait. "except if"
*We'll leave now if not you'd rather wait.

whether vs. if

subordinator takes a clause as its complement

They are sometimes interchangeable but not always. (Huddleston 11 §5.2) See Subordinate Yes/No Questions.

Works Cited

Azar, Betty Schrampfer, and Stacy A. Hagen. Understanding and Using English Grammar. 4th ed., Pearson Education, 2009.
Huddleston, Rodney D., and Geoffrey K. Pullum. The Cambridge Grammar of the English Language. Cambridge UP, 2002.
Lunsford, Andrea A., and Robert J. Connors. The New St. Martin's Handbook. 3rd ed., Bedford/St. Martin's, 1999.
Swan, Michael. Practical English Usage. 3rd ed., Oxford UP, 2005.

Practice

Road Rules

Complete the sentence with the conditional connector that is logical (+ / -).

Select the response from the list that best completes the sentence.
Compare your response to the feedback by clicking the "check" button.

Most drivers will let you change lanes you signal well in advance.

Feedback 1

You shouldn't change lanes you signal first.

Feedback 2

A driver may change lanes it is safe to do so.

Feedback 3

A driver should stop and check for oncoming traffic before making a right turn on a red light; , he will cause an accident.

Feedback 4

A driver may make a right turn on a red light it is a one-way street and the traffic is oncoming.

Feedback 5

A driver can go through a red light a police officer signals the driver to do so.

Feedback 6

A driver may drive 65 mph in a zone that is marked 65 mph it is unsafe to do so.

Feedback 7

Drivers must be careful; , accidents will happen.

Feedback 8

you put your full attention on driving, will you lower your chances of having an accident.

Feedback 9

10.

A driver should park in a handicapped space the driver has a special permit.

Feedback 10

11.

When parking, a driver should park between the marked lines. , he will take up two spaces.

Feedback 11

12.

A driver should look carefully before opening his door on the traffic side of the street, oncoming traffic may hit the door.

Feedback 12