Usage of did, were, was

When+ auxiliary verb+ subject + present verb + extension

When did you work in Chennai?

When+ auxiliary verb+ subject + extension

When were you in Chennai?

When was he in Chennai?

What usage

Do what I say.

I did what he said.

Do you understand what I say.

Do you understand what I said.

What do you say?

What did you say?

What will you say?

See, look, watch

See
On the way, I saw my old friend.

I'm going to see the doctor.

Look
Look at there, it's flying.

Look at me.

She looking at the mirror.

Whatch
I'm watching a movie.

I watched them for a long time.