Modals are verbs that do not change, and they always come before the main verb in a sentence. However, when changing the main verb tense from present to past, we see some slight differences.

How to use Modals in the Present and Past

Example

  • Present: You should see my work.
  • Past: You should have seen my work.

As you can see in these two examples, the main verb “see” becomes “seen” after changing tense from present to past. However, you should pay attention to irregular verbs (learn more about irregular verbs). Additionally, we can note that the word “have” has been added in the past tense. Therefore, modals in the past have the following form:

✏️ Modal + have + past participle. 

All modal verbs follow the above form except the ones that express obligation, ability and lack of necessity:

Examples:

Obligation:

  • Present: I must/have to study hard.
  • Past: I had to study hard.

Ability:

  • Present: I can read fast.
  • Past: I could read fast.

Lack of necessity:

  • Present: You don’t have to pay the bill.
  • Past: You didn’t have to pay the bill.
TypeModals in the PresentModals in the Past
ObligationYou must / have to stop when the traffic lights are red.You had to stop.
AdviceYou should see a doctor.You should have seen a doctor.
ProhibitionYou mustn’t smoke here.You mustn’t have smoked there.
CertaintyHe has a big house and an expensive car. He must be very rich.He must have been rich. He had a big house and an expensive car.
PermissionCan I go out? She could drive her father’s car when she was only 15.
PossibilityIt 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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