To emphasize an affirmative statement

The anomalous finites do, does and did can be placed before the verb to show emphasis. Note that after do, does and did, we use a verb in its present tense form.

I do want you to come. (More emphatic than I want you to come.)
I did invite them. (More emphatic than I invited them.)
She did accept the invitation. (More emphatic than She accepted the invitation.)

Notes on the anomalous finites

Is, am, are, was, were

These verbs are usually anomalous and their negatives are formed by the simple addition of not or n’t.

She isn’t very intelligent.
You aren’t hard working.
They weren’t expected to come.
She wasn’t anxious about it.

Interrogatives are formed by the simple inversion of subject and verb without the use of do.

Isn’t she very intelligent?
Wasn’t she anxious about it?

Has, have and had

These are anomalous when used as auxiliaries to form the present perfect and past perfect tenses.

I have seen him. Have I seen him? I haven’t seen him.
I had told him. Had I told him? I hadn’t told him.
They have disappeared. Have they disappeared? They haven’t disappeared.

Has, have and had can be used as anomalous when used as a principal verb expressing the idea of possession.

She has a car. Has she a car? She hasn’t a car.
I have a watch. Have I a watch? No, I haven’t a watch.
She has long hair. Has she long hair? No, she hasn’t long hair.

Note that the questions ‘Has she a car?’, ‘Have I a watch?’, etc., are uncommon in American English. Instead, Americans use a structure with got.

I have got a watch. Have I got a watch? No, I haven’t got a watch.

Questions and negatives made with do/does/did are also common.

Do I have a watch? (More common than Have I a watch?)

No, I don’t have a watch. (More common than I haven’t a watch.)

Note that has, have and had aren’t anomalous when they express other ideas. In this case questions and negatives are formed with do and did.

I had (= experienced) an accident.
Did I have an accident? (NOT Had I an accident?)
I didn’t have an accident. (NOT I hadn’t an accident.)
I have (= take) a bath in the morning.
Do I have a bath in the morning?
I don’t have a bath in the morning.