DO / DOES - Used simple present question and negative forms:
What time does he get up?
They don't drive to work. They take the bus.
DID - Used in simple past question and negative forms:
When did they arrive yesterday?
He didn't finish his homework last week.
IS / ARE / AM - Used in present continuous and for the future with 'going to':
They are working hard at the moment.
She is going to study medicine at university.
WAS / WERE - Past continuous:
I was watching TV when you arrived.
What were they doing while you were cooking dinner?
HAVE / HAS - Present perfect and present perfect continuous:
How long have you lived here?
I've been working since seven this morning.
HAD - Past perfect and past perfect continuous:
He had eaten by the time I arrived.
She had been studying for two hours when he finally telephoned.
WILL / WON'T - Future with 'will':
What will the weather be like tomorrow?
He won't understand.