MIXED MODALS EXERCISE 1 (CAN, SHOULD & MUST) (ESL)

Grammar review Modal verbs — ability, advice, and obligation

The three modals in this exercise each express a different type of attitude towards an action, ranging from what is possible to what is required. Getting them right is largely about understanding the strength of necessity or possibility being expressed.

Three levels of strength:
The weakest expresses ability or possibility — what someone is able or permitted to do.
The middle level expresses advice or recommendation — the right course of action, without forcing it.
The strongest expresses obligation or logical necessity — something required or almost certainly true.

Negative forms change the meaning significantly:
The negative of the “ability” modal signals that something is not allowed or impossible.
The negative of the “obligation” modal signals prohibition — not just lack of necessity.

For each sentence, ask yourself: is this about what someone is able to do, what they should do, or what they must/must not do? That question will usually point you to the right answer.