Modal verbs (can, could, may, might, will, would, shall, should, must, ought to) add meaning to the main verb — expressing ability, possibility, obligation, permission, or advice. They never change form and are always followed by the base verb.
3 subtopics — pick one to start practising
Expressing Probability
Modals express different degrees of certainty. From most to least certain: must (very sure — deduction) → should/ought to (expected) → may/might/could (possible — 50% or less). "She must be at home — her lights are on." "It might rain later." Never use "must" for future certainty — use "will" instead.
Obligation & Necessity
"Must" and "have to" both express obligation, but with a nuance: "must" usually means the obligation comes from the speaker or is internal ("I must call her — I promised"), while "have to" usually means an external rule or necessity ("I have to wear a suit for work"). "Mustn't" means it is forbidden, while "don't have to" means it is not necessary — a very important distinction!
Recommendations & Suggestions
"Should" and "ought to" give advice or recommendations. "Should" is stronger and more direct; "ought to" is slightly more formal. "Could" and "might want to" are softer, less direct suggestions. For past mistakes use "should have + past participle": "You should have called me earlier." (= but you didn't).