Dare is used both as a principal verb and as an auxiliary verb.
Dare as a principal verb
As a principal verb dare is used in the sense of defy, challenge or face boldly. Note that the principal verb dare is followed by an infinitive with to. It also has forms like dares or dared.
She dared to swim across the river.
How does he dare to do it?
Dare as an auxiliary verb
The auxiliary verb dare is followed by an infinitive without to. The auxiliary dare is common in questions and negative sentences. It doesn’t have forms like dares or dared. Questions and negatives are made without do.
He dare not do so. (NOT He dares not do so.)
She dare not take such a risk. (NOT She dares not to take such a risk.)
Dare she say that to him?
How dare he do such a thing?
The expression I dare say is no longer used with its original force. It now merely means ‘perhaps’.
I dare say he will agree to our proposal. (=Perhaps, he will agree to our proposal.)
I dare say that she is correct. (=Perhaps she is correct.)
Some equivalents of modal auxiliaries
Be able to
Be able to is often used instead of can or could.
I am able to help her. OR I can help her.
She can do this. OR She is able to do this.
The police could catch the thief. OR The police was able to catch the thief.
The structure be + to is often used to indicate simple future like will or shall.
She is to arrive by the evening flight. (=She will arrive by the evening flight.)
We are to visit them tomorrow. (=We will visit them tomorrow.)
Note that the structure be + to shows simple futurity with a slight uncertainty. When we say She is to arrive, we are not quite certain whether she will actually arrive or not.
Sometime be + to is used to express a command.
You are to finish that work in two days.
He is to report for duty within a week.
Had better has similar meaning to should and ought.
You had better get some rest. (=You should get some rest.)
He had better be careful. (=He should be careful.)