According to the syntactic function of verbs, which depends on the extent to which they retain, weaken or lose their meaning, they are divided into notional verbs, auxiliary verbs and link verbs.

Notional verbs are those which have a full meaning of their own and can be used without any additional words as a simple predicate. Here belong such verbs as to write, to read, to speak, to know, to ask.

Ricky surrounded her with great care and luxury. (Stern)
She knew what he was thinking. (Galsworthy)

Auxiliary verbs are those which have lost their meaning and are used only as form words, thus having only a grammatical function. They are used in analytical forms. Here belong such verbs as to do, to have, to be, shall, will, should, would, may.

I don’t recollect that he ever did anything, at least not in my time.(Galsworthy)
Their father… had come from Dorsetshire near the beginning of the century.(Galsworthy)
But all this time James was musing... (Galsworthy)
He would have succeeded splendidly at the Bar. (Galsworthy)ear of it. (Prichard) (MODAL VERB)

Link verbs are verbs which to a smaller or greater extent have lost their meaning and are used in the compound nominal predicate.

The house was too big. (Galsworthy)
The old face looked worn and hollow again. (Galsworthy)
Manson no longer felt despondent, but happy, elated, hopeful. (Cronin)

In different contexts the same verb can be used as a notional verb and an auxiliary verb or a link verb:

…She turned her head sullenly away from me. (Collins) (NOTIONAL VERB)
She… turned deadly pale. (Collins) (LINK VERB)
No one was there to meet him. (Lindsay) (NOTIONAL VERB)
She was not a ten-year-old girl any more… (Dreiser) (LINK VERB)
This evening Bathsheba was unusually excited, her red cheeks and lips contrasting lustrously with her shadowy hair. (Hardy) (LINK VERB)

There is a special group of verbs which cannot be used without additional words, though they have a meaning of their own. These are modal verbs such as can, may, must, ought, etc.

A slow swell of feeling choked the little boy’s heart. Though he could not, dared not question the consul’s strict command, its purpose lay beyond his comprehension. (Cronin)
“We ought to have stayed in Italy,” he said. “We ought never to have come back to Manderley.” (Du Maurier)

The same verb in different contexts can be modal and auxiliary.

I crouched against the wall of the gallery so that I should not be seen. (Du Maurier) (AUXILIARY VERB)
I don’t honestly think Lady Crowan was exaggerating when she said something should be done in your honour. (Du Maurier) (MODAL VERB)
I had no idea she would do that. (Du Maurier) (AUXILIARY VERB)
He needed a cook. Why couldn’t she apply for the job? But Morris would not hear of it. (Prichard) (MODAL VERB)