Modal Verbs: Ability & Advice
can, could, should for ability and advice.
Modal verbs are special helping verbs that we use before a main verb to talk about ability, permission and advice. We use 'can' to talk about ability in the present (I can swim) and to ask for or give permission (Can I go out? You can sit here), and we use 'could' for ability in the past (When I was five, I could read) and for polite requests (Could you help me?). We use 'should' to give advice or to say that something is a good idea (You look tired. You should rest). A very important rule is that after a modal verb we always use the bare infinitive, which is the base verb with no 'to' (She can swim, NOT She can to swim). Modals do not add -s in the third person, so we say 'He can play', not 'He cans play'. We make negatives by adding 'not' to the modal: cannot/can't, could not/couldn't, should not/shouldn't (You shouldn't eat so much sugar). The same modal form is used for every subject, so it never changes. For example: 'You should drink water, but you can't drink the river water because it is dirty.'
Key terms
| Modal | Meaning | Example |
|---|---|---|
| can | present ability | I can swim very well. |
| could | past ability | When I was five, I could read. |
| should | advice (a good idea) | You look tired. You should rest. |
| shouldn't | negative advice (a bad idea) | You shouldn't eat so much sugar. |
| Form | Rule | Example |
|---|---|---|
| Affirmative | modal + base verb (bare infinitive), no 'to' | She can play the piano. |
| Third person | modal never adds -s; same for every subject | He can run, NOT He cans run. |
| Negative | modal + not: can't, couldn't, shouldn't | You shouldn't be late. |
| Question | modal + subject + base verb | Can I open the window? |
- 1Read the sentence: You look tired. You ____ rest.
- 2Decide: ability or advice?: The sentence recommends a good idea for a tired person. That is advice, not ability, so we need 'should', not 'can'.
- 3Check the verb form: After the modal we use the bare infinitive: should rest (no 'to', no -s).
- 4Answer: You should rest.
- 1Read the sentence: When he was a baby, he ____ not walk.
- 2Find the time clue: 'When he was a baby' shows the past. For past ability we use 'could', not the present 'can'.
- 3Build the negative: Add 'not' to the modal: could not / couldn't, followed by the base verb 'walk'.
- 4Answer: When he was a baby, he could not walk.
Never put 'to' after a modal. Write 'She can play' and 'He should do his homework', NOT 'She can to play' or 'He should to do'.
Modals never add -s in the third person. Say 'He can run' and 'She should drink', NOT 'He cans run' or 'She should drinks'. The base verb stays the same for every subject.
'could' is the past of 'can'. Use 'could/couldn't' for the past (When I was younger, I could run fast) and 'can/can't' for the present (now I can swim).
Don't mix up ability and advice. 'can/can't' are about ability or permission, while 'should/shouldn't' give advice. To recommend something use 'should'; to say it is a bad idea use 'shouldn't'.
Rules
- 1Use 'can' for present ability and permission (I can dance; Can I leave?), and 'could' for past ability and polite requests (I could swim at six; Could you wait?).
- 2Use 'should' to give advice or say what is a good idea (You should study more; You shouldn't be late).
- 3After a modal, always use the bare infinitive (base verb with no 'to'): 'She can swim', NOT 'She can to swim'.
- 4Modals never add -s in the third person and stay the same for every subject: 'He can run', NOT 'He cans run'.
- 5Make negatives by adding not: can't (cannot), couldn't (could not), shouldn't (should not).
Practice
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