Purpose
By the end of this lesson, you will be able to correctly use the modal verbs “can,” “must,” and “should” to express ability, obligation, and advice.
Lesson Explanation
Modal verbs add a specific meaning to another verb and never change form, regardless of the pronoun:
can — ability or possibility: I can swim. must — strong obligation: You must wear a seatbelt. should — advice or recommendation: You should see a doctor.
A modal verb is always followed directly by the base form of the main verb — never -ing, never to: I can swim (correct), not “I can to swim” or “I can swimming.”
To make a modal negative, add not directly after it: cannot (can’t), must not (mustn’t), should not (shouldn’t).
Must not means something is forbidden; this is genuinely different from don’t have to, which means something is optional, not forbidden.
Practice Questions
1. Complete: “I ___ swim.” (ability)
View Answer
can
2. Complete: “You ___ wear a seatbelt.” (strong obligation)
View Answer
must
3. Complete: “You ___ see a doctor.” (advice)
View Answer
should
4. Does a modal verb change form for “he/she/it,” like regular verbs do?
View Answer
No — modal verbs never change form.
5. What comes directly after a modal verb: the base verb, or the -ing form?
View Answer
The base verb.
6. Is “I can to swim” correct or incorrect?
View Answer
Incorrect — no “to” after a modal verb.
7. Write the negative (contracted) form of “must.”
View Answer
mustn’t
8. Write the negative (contracted) form of “should.”
View Answer
shouldn’t
9. What does “must not” mean?
View Answer
Something is forbidden.
10. What does “don’t have to” mean, and how is this different from “must not”?
View Answer
It means something is optional, not forbidden — genuinely different from “must not.”
11. Complete: “You ___ smoke here.” (forbidden)
View Answer
must not
12. Write the negative (contracted) form of “can.”
View Answer
can’t