Need, dare, and used to are considered "semi-modal verbs" because they function in much the same way as modal verbs - they are an auxiliary verb that adds information to the main verb, but can't really function alone.

They are different from modal verbs, though, because they behave more like typical verbs - for example, they change to agree with their subjects, they change tense, and they can be combined with other helping verbs like be, have, and do.

Need is used...

USES

EXAMPLES

To communicate necessity or obligation. In this context, "need" is a semi-modal verb, and we must use "to" before the main verb.

We need to go grocery shopping.

In formal negative constructions, need is often used without "to" before the main verb. In this context, need is used as a modal verb, and you do not need to include "to" before the main verb. [Note: this construction is used more often in British English.]

They needn't go to such trouble.

Dare is used...

USES

EXAMPLES

Dare is often also used in formal questions.

Do they dare come to the wedding without formal dress clothes?

Like "need," dare is more often used in formal negative sentences, and more often in British English.

I don't dare come late to class!

To show that the subject is making a conscious effort, or has great desire to do the action of the main verb.

I don't know how he dares behave so rudely in public!

Used to is used...

USES

EXAMPLES

To convey the idea that an action happened habitually in the past, but no longer occurs.

We used to walk to school one mile each way!

With be as a helping verb, used to gives the idea of being accustomed to doing the action of the main verb.

They're used to watching the nightly news over dinner.

"Ought to" -- both a modal verb and a semi-modal verb

"Ought to" is a bit of a special case. It has a lot of tendencies of modal verbs - it doesn't ever change forms or use another helping verb.

But, it looks more like a regular verb because you have to include "to" before the main verb.

It just goes to show you, there are always exceptions to the rules!

For example:

She ought to spend more time practicing piano.

You can see elements here of both modal and semi-modal verbs. Like a modal verb, you don't add an -s to ought, even though she is in the third person singular. Like a semi-modal verb, you follow ought with to.

We ought to go to the beach next week.

Again, you can see elements here of both modal and semi-modal verbs. Like a modal verb, you don't add will before ought, even though we are discussing the future. Like a semi-modal verb, you follow ought with to.

Summary

Semi-Modal Verbs function in much the same way as modal verbs - they are an auxiliary verb that adds information to the main verb, but can't really function alone.
They are different from modal verbs, though, because they behave more like typical verbs - for example, they change to agree with their subjects, they change tense, and they can be combined with other helping verbs like be, have, and do.
The main verb that follows a semi-modal verb is always the "bare infinitive form" - the basic verb form. Remember, that with semi-modals, "to" is part of the verb phrase.
Need communicates necessity or obligation. In formal negative constructions, need is often used without "to" before the main verb.
Dare shows that the subject is making a conscious effort, or has great desire to do the action of the main verb. It is more often used in formal negative sentences, and more often in British English. Dare is often also used in formal questions.
Used to conveys the idea that an action happened habitually in the past, but no longer occurs. Also, with be as a helping verb, used to gives the idea of being accustomed to doing the action of the main verb.
"Ought to" has tendencies of both modal verbs and semi-modal verbs. Like modal verbs, it doesn't ever change forms or use another helping verb. But, like a regular verb, you have to include "to" before the main verb.