2.- How to say it. Modal verbs referring to the past (I).

All these modal verbs go with a present infinitive and refer to the present or future time.

• Advice about the present time: She should get up early.
• Ability at present time: He can learn languages easily.
• Permission at present time: You can have the morning break now. It is nearly midday.
• Deduction about the present: They must have several children because there are lots of toys in the house.

Study these sentences:

• You should study for your exam yesterday.
• If you can't find your gloves anywhere, you could leave them in the cinema last night.

What is wrong with them? There is a grammar mistake.

• In English and Spanish, when we want to refer to the present or future time, we use a present infinitive.
• However, when we want to refer to the past time, we normally use a perfect infinitive.

How can we form the perfect infinitive? It is quite easy: the auxiliary have + past participle.

• You should have studied for your exam yesterday.
• If you can't find your gloves anywhere, you could have left them in the cinema last night.

Then, the modal verb structure to refer to the past is:

Subject + modal verb + have + past participle, as in You may have studied this before.