I could swim when I was 6
I have to study chemistry
You should go to see your doctor
May I take your pen?
Could you help me?
I can't do it.
You needn't hurry
He had to come back
They have to work a lot
You must learn it by heart
Shall we wait here?
They ought to tell him the truth as soon as possible.
She might have sent us an e-mail.