Modals in the Present and Past

Modals in the present and past

Generally speaking modals in the past have the following form:

• modal + have + past participle

Example:

• Present:
  You should see a doctor.
• Past:
  You should have seen a doctor

Except for modals that express obligation,ability and lack of necessity:

• Obligation:
  Present = I must / have to work hard. -- Past = I had to work hard.
• Ability:
  Present = I can run fast. -- Past = I could run fast when I was young.
• Lack of necessity:
  Present = You don't have to / needn't take your umbrella. -- Past = You didn't have to / didn't need to take your umbrella.

	Modals in the Present	Modals in the Past
Obligation	You must / have to stop when the traffic lights are red.	You had to stop.
Advice	You should see a doctor.	You should have seen a doctor
Prohibition	You mustn't smoke here.	You mustn't have smoked there.
Ability	I can run fast.	I could run fast. now I am old.
Certainty	He has a Rolls Royce. He must be very rich. He can't be American. His English is terrible.	He must have been rich. He had a big house and an expensive car. He can't have written that poem. He was illiterate.
Permission	Can I go out?	She could drive her father's car when she was only 15.
Possibility	It may / can / could / might rain. It's cloudy.	I guess it may / can / could / might have been Lacy on the phone.
Lack of necessity	You don't have to / needn't buy any tomatoes. There are plenty in the fridge.	You didn't have to / didn't need to buytomatoes.