The words will, would, shall, should, can, could, may, might, must and ought to are called modal auxiliary verbs. Sometimes the verbs dare and need are also considered as modals. Unlike primary auxiliaries which have distinct forms to refer to the past, the modals do not have past forms.
The modal auxiliary verbs usually refer to the present or future. Study the following sentences.
- She can knit. (This sentence refers to ability in the present.)
- She may come. (This sentence refers to probability in the future.)
- She must come. (This sentence refers to obligation in the present or future.)
In some cases, the auxiliary verbs could, should, would and might can be used to refer to the past. This usually happens in reported speech.
- She said, “I will come.”
- She said that she would come.
- She said, “I can walk four miles at a stretch.”
- She said that she could walk four miles at a stretch.
Note that these aren’t strictly past tense auxiliaries and sometimes they refer to the present or future. For example, the auxiliaries could and would can refer to the present or future in polite requests. Similarly, might can show less probability in the future.
- Could you come here, please?
- Would you mind waiting outside?
- Could you move a bit?
- Would you open the window?
Should usually shows obligation and then it refers to the present or future.
- She should mend her ways.
- They should send the parcel today itself.
Should can be the past tense of shall in indirect speech.
- He shall pay the fine.
- The magistrate ordered that he should pay the fine.