An ELT Glossary : Semi-Modal verbs
Need, dare and ought are semi-modal verbs. They are called this because they are verbs which can act either as a modal verb - ie as an auxiliary, which can act as the operator and is followed by a bare infinitive :
We needn't finish it today
I daren't tell him.
or as an ordinary lexical verb using the dummy operator DO and followed by to + Infinitive:
We don't need to finish it today.
I don't dare touch it.
There are however restrictions on their use as modal auxiliaries - they tend only to be used as like this in non-assertive contexts as in the examples above. If a proposition is being asserted, the lexical form must be used - eg We need to finish it today and not *We need finish it today.
Ought is also seemingly strange in that it a) it uses to even when acting as operator :
He oughn't to be here.
and uses did as operator rather than do when used as a lexical verb:
Did he ought to be here?
There is, however, an etymological reason for the latter difference. Historically ought is the irregular past form of owe. As the language has developed, the meaning has changed, but the grammar has remained.