What does the baby need to do?
He needs to sleep.
What does he have to do?
He has to / must buy some food; He must go shopping.
What does he need to do?
He needs to learn how to ride a bike / He needs to buy a new bike.
What does he have to do?
He has to / must be careful when crossing the street.
What does she have to do?
She has to / must throw away the milk; She must buy some milk.
That joke ... be really funny
have to
mustn't
need to
must