The infinitive is the basic form of a verb. The infinitive form has to before the verb (e.g. to think, to understand, to throw, etc.). When there is no 'to' before the verb, we call it bare infinitive (think, understand, throw, etc.). We will use the infinitive or the bare infinitive depending on the previous verb:

Bare infinitive

We use the bare infinitive after will, must, should, can, ought to, would, may, might:

• I will try. I will to try.
• I think she can play the flute. I think she can to play the flute.
• We must stay here. We must to stay here.

We need to use the bare infinitive after some verbs, such as need (except when it is the main verb), make, let, see, watch, hear, etc:

• Let me try it. Let me to try it.
• We saw her go. We saw her go.

The bare infinitive is also used with the expressions better, had better, would rather, rather than and sooner than:

• I would rather go now. I would rather go now.

Infinitive

We use the infinitive after many verbs: need, learn, like (especially in American English), talk, want, would like, etc:

• I need to eat something.
• I would like to go to bed.