I've (I have) seen him.

I haven't (have not) seen him. 

Have you seen him?

He's (he has) gone.

He hasn't (has not) gone.

Has he gone?

I saw him yesterday.

I didn't (did not) see him yesterday.

Did you see him?

I worked in London.

In positive sentences, a helping verb such as 'was' or 'did' is not used.

Did he work in London?

The helping verb 'did' is used in past simple questions.

Did he work in London?

The helping verb 'did' is used in past simple questions.

Did he write a letter?

The main verb is used in the infinitive form in questions and negatives.

He didn't write a letter.

The main verb is used in the infinitive form in questions and negatives.

Steven has written a new book.

The past participle of the verb must be used - wrote is past simple, written is the past participle.

Have you seen him before?

The helping verb 'have' is used in the present perfect- it is inverted with the person (you have becomes have you).

I haven't seen him before.

The helping verb 'have' is used in the present perfect- to make it negative we simply add not (n't).

I have been here since last week.

The present perfect is used to show an action which continues to the present (an unfinished action).

I've known him for 5 years.

Verbs such as know, want, like, etc. (stative verbs) suggest permanent states, not actions, so are used in the simple form, NOT the -ing form.