The infinitive is a particular verb form which expresses the verbal idea in its simplest form. It has no marking for tense, mood, person or other grammatical categories.
In English, the infinitive is the bare form of the verb. It is the form which can immediately follow a modal auxiliary verb like will or may. Examples are: break, stop, invite, work, write, decide etc.
The infinitive can follow the particle: to. This form is sometimes called the to-infinitive. Note that that to is not a part of the infinitive at all; it can readily be separated from the following infinitive by a that-clause.
Changes in the form of a verb for grammatical reasons. The noun book has two inflected forms: book and books. The verb write has five inflected forms: write, writes, writing, wrote, written. In the same way, the verb be has three inflected forms: be, being and been.
A word or phrase which, as a rule, occurs by itself. An interjection expresses emotion. It does not form part of a sentence. English examples are: Damn!, Ouch!, My God!
A pronoun which asks a question. The English ones are: who, what and which.
Who were you talking to?
What are you doing there?
Which do you prefer?