Primary (in finite clauses) vs. Secondary (in nonfinite clauses)
PRIMARY VERB FORM
A primary verb can be marked for tense (walked), person (walks) and in some cases number (is, was, were). Present and past tenses are formed with verb inflection, the other tenses are formed in combination with auxiliary verbs. A finite clause includes a primary verb.
We walk the dog everyday.
She walks the dog everyday.(marked for 3rd per sing.)
Our dog tires easily.
We walked the dog everyday.(marked for tense)
She walked the dog everyday.
The trip tired us.(marked for tense)
SECONDARY VERB FORM
Asecondary verb is not marked for tense, aspect, mood, number and person, and it cannot serve as a predicate, nor can it be used in an independent clause. See Nonfinite. (These are also called "reduced verbs".) A nonfinite clause includes a secondary verb.
PLAIN FORM / INFINITIVAL
He can walk. (plain form)
The dog makes us walk faster. (plain form, infinitive without "to")
He wants to walk. (infinitive with "to")
She is walking in the rain. (aux. complement, progressive)
She likes walking in the rain. (verb complement)
People walking their dogs seem happy. (noun complement–modifier)
They rested afterwalking for two hours.(preposition complement)
PAST PARTICIPLE FORM
The dog has tiredits owners. (aux. complement, pres. perfect)
The dog was tired by walking so much. (aux. complement, passive)
The tiredwalkers rested their feet. (past participle modifier)
The peopletired of walking took a taxi home. (past participle modifier)
Primary forms are inflected for mood or tense; secondary forms are the remainder. (Huddleston 3 1.8)
A bare infinitive (plain or bare verb form) without to is used after dare, need, help and modals.
A finite clause may stand alone as a complete sentence. It includes a subject and a primary verb form that can be inflected (suffixed) for tense, person and sometimes number. It is an independent clause (matrix clause). See Finite / Nonfinite and see Grammar notes.
A nonfinite clause cannot stand alone. It rarely includes a subject, and its verb is a secondary verb form (infinitival, gerund-participle or past participle) which cannot be inflected for tense, person or number. It is a dependent clause serving as a subject or a complement to a verb, preposition or noun.
Most verbs are lexical verbs. A lexical verb has a dictionary meaning and can be marked for tense and 3rd person. It normally uses do support for questions, negatives and emphasis.
MEANING AND TENSE
You have a computer. (present) (possession)
You had a computer. (past)
DO & INVERSION—QUESTIONS
Do you have a new computer?
DO + NOT—NEGATIONS
You do not like your new computer.
DO—EMPHASIS / TAG QUES. / PAIRED CONJ
I do like my new computer!
You like your new computer, don't you?
He likes his new computer, and you do too?
A non-modal auxiliary can be marked for tense and 3rd person, but it does not have a dictionary meaning. It combines with a lexical verb to form meaning. It uses auxiliary support for questions, negatives and emphasis.
TENSE BUT NO MEANING
He isworking on his computer. (is, was)
She hasworked on her computer. (has, had)
She willwork on her computer.
AUX & INVERSION—QUESTIONS
Is he working on his computer? (Aux ← Subj)
Has she worked on her computer? (Aux ← Subj)
Will she work on her computer?
AUX + NOT—NEGATIONS
He is notworking on his computer. (is, was)
She has notworked on her computer. (has, had)
She will not work on her computer.
AUX—EMPHASIS / TAG QUES. / PAIRED CONJ
He is working on his computer!
She has worked on her computer, hasn't she?
She will work on her computer, and so will you.
Both lexical and auxiliary verbs are considered primary (finite) verbs – inflected with tense and person. (Swan 85)
Biber sets out the classes as full (lexical verbs) primary (be, have, and do) and modal auxiliaries (will, might, etc.)
(Biber 3.21, 96)