1. Form
The infinitive is the base form of a verb. It may be preceded by 'to' (the to-infinitive) or stand alone (the base or zero infinitive).

2. Infinitive with or without 'to'

The to-infinitive is used:

a. after certain verbs. e.g. want, wish, agree, fail, mean, decide, learn
b. after the auxiliaries to be to, to have to, and ought to
c. in the pattern 'it is + adjective + to-infinitive'

Examples:

with 'to'

• The elephant decided to marry the mouse
• The mouse agreed to marry the elephant
• You will have to ask her
• You are to leave immediately
• He ought to relax
• She has to go to Berlin next week
• It's easy to speak English
• It is hard to change jobs after twenty years
• It's stupid to believe everything you hear

without 'to'

• I would rather visit Rome.
• She would rather live in Italy.
• Would you rather eat steak or fish?
• He would rather work in a bank.
• I'd rather be a forest than a tree.