How modals operate in the present and the 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
can run fast.
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 buy tomatoes.

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