Grammar-Quizzes › Verb Phrases › Verb Groups › Modals › Can vs. Could

Can vs. Could

Express physical, mental and potential ability

General/Physical vs. Mental Ability

PHYSICAL ABILITY
Can expresses opinion about general or physical ability. Something or someone is able to do something. (strong or powerful enough, capable of)
CAN / COULD	PLAIN FORM VERB
"Watson", a super computer, can	answer questions in "natural language".
Watson can	access 200 million pages of information in seconds.
Watson cannot	think like a human. It can only learn.
In most cases, the computer could	win. (come up with correct answers first)

MENTAL ABILITY
Can also expresses opinion about mental ability. Something or someone knows how to do something.
CAN / COULD	PLAIN FORM VERB
Ken Jennings can	speak on a wide variety of subjects.
Ken can	play a game to win.
Ken could	outsmart his opponents every time.
Often, Ken could	outsmart Watson's reasoning ability.

Watson is an artificial intelligence computer system capable of answering questions posed in natural language, developed by IBM. http://en.wikipedia.org/wiki/Watson_(computer)

Jeopardy – a game show in which contestants are give the answer and must come up with the question

Also see May / Can (permission)

Be Able / Can / Could

Past success and future potential

A single past event – was/were able to

REACHED A GOAL
Was / were able to express opinion about reaching a particular goal or success. Something or someone managed to do something. (single event–past)
WAS / WERE ABLE	INFINITIVAL VERB FORM
IBM engineers were able (managed)	to design a computer that uses "natural language".
After several tries, IBM engineers were able	to program Watson to win a match.
Ken Jennings was able to	win 75 Jeopardy matches.
Watson was not able	to understand his opponent's answers.

POTENTIAL
Can expresses opinion about potential ability. Something or someone has the potential or is likely to do something. Both can and could express future potential.
CAN / COULD	PLAIN FORM VERB
In the future, Watson can	help suggest treatment options to doctors.
Watson can	analyze a patient's symptoms and medical history.
Ken Jennings can	use his celebrity to endorse products.
One day, IBM could	build a computer with cognitive power.

cognitive (Adj) – having mental processes of perception, memory, judgment, and reasoning,

contestant (N) – person playing or competing in a game or competition

opponent (N) – the person you play against in a game.

Could / Was Able

Express past ability in general or in particular

Could vs. Was/Were able

COULD – GENERAL ABILITY
Could is used for ability with a general sense of time. Could is not used for a moment of completion or success, a particular and singular time pinned to the past . It expresses a "timeless" ability.
MULTIPLE PAST EVENTS OR ONGOING
I practiced a lot during the winter. By summer, I could drive pretty well. (general timing) If I wanted to, I could drive to work or to the store.
Before 1935, anyone could drive. Now the state requires a license. Only state residents could apply for a driver's license.

WAS ABLE – PAST ABILITY
Was/Were able expresses ability with a particular and singular time pinned to the past, a moment of completion or success. A similar meaning is expressed by "managed " + infinitive or "succeeded in" + gerund.
SINGLE PAST EVENT – MOMENT OF COMPLETION
I practiced a lot during the winter. By summer, I was able to pass my driving exam. By summer, I could pass my driving exam. (single event)
I was able to get my driver's license. I managed to get my driver's license. I succeeded in getting my driver's license.

Eng-US: license (n./ v.) , Eng–Br: licence (N); Eng-US: practice (n./ v.); Eng–Br: practise (V)

pop question Pop-Q Resultive "so".

Sentence Types

Word Order

MODAL	SUBJECT	MODAL / VERB EXPRESSION	MAIN VERB	OBJECT PHRASE
STATEMENT
	Jack	can	cook	dinner.
	Jack	is able to	fix	your bicycle.
	Jack	knows how to	use	a computer.
QUESTION
What can	Jack		do?
Can	Jack		lift	that computer by himself?
Is	Jack	able to	drive	your car?
Does	Jack	know how to	use	a computer?
NEGATIVE
	Jack	can't	come	home early.
	Jack	is not able to	come	home early.
	Jack	doesn't know how to	come	home early.
PAST
	Jack	could	stay up	all night when I was 18.
	Jack	was able to	stay up	all night on New Years.
	Jack	knew how to	use	a computer
WTH AN ADVERB
	Jack	can usually / usually can	cook	dinner in an hour.
	Jack	is sometimes able to	do	his homework.
	Jack	always knows how to	fix	a computer problem.
W/ NEG. ADVERB
*Seldom can	Jack		get	his homework done.

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Grammar Notes (Advanced)

Traditional and Linguistic Description

Traditional / ESL and Linguistic Descriptions

TRADITIONAL & ESL DESCRIPTION	LINGUISTIC DESCRIPTION
AZAR	HUDDLESTON
can / could (Azar 10.1-4) possibility modal → He could be sick. (possibility – 50% degree of certainty); past modal modal → He could have been sick. (possibility – < 50% degree of certainty) negative modal → He couldn't be sick. (possibility – 99% degree of certainty) past modal → Tom could lift the box. Tom said he could speak French. (ability) polite request → Could you help me? suggestion (an option; one of several ideas) → You could ask a different doctor. (20-4)	can / could "Mood and Modality" (Huddleston 3 §9) possibility (epistemic) → weak commitment → He can't have done it.(neg. only) / He could be the killer. [possibility] possibility (deontic ) → He can have one more cookie. [permission] possibility (dynamic) (3 §9.3.3) You can always say "no". [reasonable/ acceptable] Rain can get in. [circumstantial possibility] Dogs can bite. [existenitial] He could set a record today. [potential ability] He can / could speak Swahili. [actualized ability] backshift → He said he could speak Swahili. [actualized ability] He thought that Valjean could be the killer. [possibility] polite form–distancing ("modally remote preterite") → Could you help me? (3 §9.8.3)
SWAN	BIBER
can/ could (Swan 122-5) ability present → I can read Italian. (knowledge, skill, strength) past → I could read quickly. He said he could read Italian. After studying a year, I was able to read Italian. (122.5) common or typical → It can be hot here in summer. possible in the situation → We can go to Athens. soft suggestion → We could try a different approach. unrealized past → We could have gone to Athens, but we didn't chances (possibility) → It could rain this afternoon. (122.8) permission → Could we have some cake? (more polite than can) (124) requests, orders, and suggestions → Could we have some cake? (124.6) criticism → You could have told me you were dating my girl. indirect speech → I asked if they could give me some help.	can / could (Biber 6.6.4.1) permission / possibility / ability: can, could Could and might are much more common expressing logical possibility than permission or ability. In contrast to the typical functions of can, the modal could usually marks logical possibility in conversation, expressing a greater degree of uncertainty or tentativeness. That could be her. It could be anything you choose. Epistemic Stance.modal Verb in extrinsic sense → Without… collaboration there could be interference… (973)

Works Cited

Azar, Betty Schrampfer, and Stacy A. Hagen. Understanding and Using English Grammar. 4th ed., Pearson Education, 2009.
Biber, Douglas, and Stig Johansson, et al. Longman Grammar Of Spoken And Written English. Pearson Education, 1999.
Huddleston, Rodney D., and Geoffrey K. Pullum. The Cambridge Grammar of the English Language. Cambridge UP, 2002.
Swan, Michael. Practical English Usage. 3rd ed., Oxford UP, 2005.