Use can, could, must, should and more to express ability, obligation, and possibility.
Modal verbs are helping verbs. They come before the main verb and do not change for he, she, or it. Use can and could for ability. Can shows ability now. Could often shows past ability or a more polite form. • I can swim. • She could read when she was five. Use can, may, and could for permission. Can is common in speaking. May is more formal. Could is polite. • Can I open the window? • May I leave early? Use will and would for requests and offers. Will is direct. Would is more polite. • Will you help me? • Would you like tea? Remember: modal verbs do not need s in the third person. We say he can, not he cans. They also do not use do or does in questions. • Can you help me? not Do you can help me? • She can dance. not She cans dance.
• I can speak English. (ability now) • He could play the piano when he was young. (past ability) • May I sit here? (polite permission) • Can you pass the salt? (simple request) • Would you like some water? (polite offer)
Use can for simple, common questions. Use could and would to sound more polite. Modal verbs stay the same with he, she, and it.
Modal verbs can show rules, duty, and advice. Use must and have to when something is necessary. Must is often strong and personal. Have to is often a rule or outside duty. • You must wear a seat belt. • I have to work today. Use should and ought to for advice. They mean something is a good idea. • You should drink water. • You ought to rest. Use must not and cannot for prohibition. They mean something is not allowed. • You must not run here. • You cannot smoke in this room. Need to is also common for necessity. It is similar to have to. • I need to study tonight. At beginner level, remember the basic meaning: • must and have to = necessary • should and ought to = good advice • must not and cannot = not allowed Modal verbs are useful because they show the speaker's idea clearly and quickly.
• You must stop at the red light. (strong rule) • I have to go now. (necessity) • You should see a doctor. (advice) • You must not touch that. (prohibition) • We need to buy milk. (need)
Think of must as strong and should as helpful advice. Use must not for something that is not allowed.
Modal verbs can show how sure we are about something. May and might show possibility. Will can show strong certainty or a future fact. Must can show a very strong guess. • It may rain today. • It might rain later. • She will be at home now. • He must be tired. Degree of certainty is important. May and might are less certain. Will is more certain. Must is the strongest guess. Modal verbs can also talk about the past. • could and was able to can show past ability. • may have, might have, could have, must have are common for past guesses. • He could run very fast as a child. • They might have missed the bus. • She must have forgotten her keys. Common errors: • Do not say He musts go. Say He must go. • Do not use to after every modal. Say I can swim, not I can to swim. • Do not add ed to modals. Say could, not coulded. A good way to learn modals is to ask: Is it ability, permission, advice, duty, or a guess?
• It may snow tomorrow. (possible) • She might be late. (less certain) • He will come soon. (very likely) • They must be at school now. (strong guess) • She might have called you. (past possibility)
Use may and might for weak possibility. Use must be or must have for a strong guess.