Can — Ability & Permission
Use 'can' to talk about ability (what you know how to do) and to ask for or give permission.
Must — Obligation & Necessity
Use 'must' to talk about rules, strong obligations, and things that are necessary. 'Must not' (mustn't) means it is forbidden.
Should — Advice & Recommendation
Use 'should' to give advice, opinions, and recommendations. 'Shouldn't' means it's not a good idea.
Grammar Note
Modal verbs are followed by the base verb (no -s, no -ing, no 'to'). They don't change for he/she/it.
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Modal verbs — can, could, may, might, must, should, will, would, shall — add meaning about ability, permission, obligation, and possibility. They are followed by the base verb and never take 's' or 'ed'.
Ability and Permission
'Can' shows ability or informal permission: 'I can swim', 'Can I sit here?'. 'Could' is the past of ability or a polite request: 'Could you help me?'. 'May' is formal permission or possibility: 'May I leave early?' and 'It may rain'. Choosing the right modal changes how confident or polite you sound.
Obligation and Advice
'Must' is strong obligation or certainty: 'You must wear a seatbelt.' 'Should' is advice or expectation: 'You should see a doctor.' 'Have to' is external obligation. Mixing 'must' and 'should' changes the force of your message, so practice them in context to sound natural.
Possibility and Prediction
'Might' and 'may' express possibility; 'will' predicts; 'would' is hypothetical or polite. 'I would travel if I had time' is second conditional. These modals appear in almost every conversation about plans, so saying them aloud builds fluency fast.
Common Mistakes to Avoid
Modals break normal verb rules, so learners add '-s' ('he cans'), use a gerund after them ('can to go'), or pile two modals ('will can'). Confusing 'must' and 'should' changes how forceful you sound, and using 'can' for past ability is wrong (say 'could'). Remember modals are always followed by the bare base verb and never take 'to', '-s', or '-ed'. Keeping them straight is what makes your requests and advice sound natural rather than robotic.
Try This: A Quick Practice
Write three ability sentences ('I can swim'), three advice sentences ('You should rest'), and three permission sentences ('May I sit?'). Say them aloud and notice the shift in tone from ability to advice to politeness. Then turn one present ability into the past: 'I could swim as a child'. SpeakNow's modal-verbs exercise repeats these patterns out loud so the base-form rule sticks without conscious effort.
Frequently Asked Questions
What are the nine modal verbs?▼
Can, could, may, might, must, shall, should, will, would. They are always followed by the base form (e.g., 'she can swim') and never add -s or -ed.
What is the difference between 'must' and 'should'?▼
'Must' is strong obligation or near-certainty ('You must stop'). 'Should' is softer advice or expectation ('You should rest'). The difference in force matters in polite conversation.
Why don't modals take 's' in third person?▼
Modals are defective verbs with no infinitive or -s form; 'he can' not 'he cans'. This is a fixed rule worth practicing out loud on SpeakNow.