The infinitive is always used without to after the auxiliaries can, could, may, might, must, will, would, shall, should, do, does and did.
- She can swim. (NOT She can to swim.) (NOT She can swimming.)
- He must obey me. (NOT He must to obey me.) (NOT He must obeying me.)
- She should understand. (NOT She should to understand.) (NOT She should understanding.)
The modal auxiliary ought is an exception to this rule. It is followed by an infinitive with to.
- She ought to behave. (NOT She ought behave.) (NOT She ought behaving.)
The primary auxiliaries be (is, am, are, was and were) and have (has, have and had) can be followed by an infinitive with to.
- She is to retire next year. (NOT She is retire next year.)
- He has to pay the fine. (NOT He has pay the fine.)
The modal auxiliaries need and dare can be followed by an infinitive with or without to. The grammar is different.
In questions and negatives need is usually followed by an infinitive without to. In affirmative sentences, need is usually followed by an infinitive with to.
- Need I wait any longer?
- Need I consult a specialist?
- You need not wait any longer.
- You need not consult a specialist.
- You need to wait for an hour or two. (More natural than ‘You need wait for an hour or two.’)
- You need to consult a specialist.
When need is followed by an infinitive with to, we make questions and negatives with do.
- You need to sign these papers.
- Do I need to sign these papers? OR Need I sign these papers? (NOT Need I to sign these papers?)
- You don’t need to sign these papers. OR You need not sign these papers. (NOT You need not to sign these papers.)