They are self-sufficient and do not require auxiliaries in questions and negative sentences.
They do not require TO (except have to, ought to and need to):
In the Present Simple they don't require -S in the third person singular.
· Can you swim? Do you can swim?
· May I come in?
· You shall not pass!
· I wouldn't let.
· They will to fight.
· You should to leave.
· She can speak English.
· He must apologise.
Keep that in mind. Many students tend to say "must to / should to", which is a mistake.