Grammar topic
Modal verbs: ability, rules, advice and guesses
Modal verbs add the speaker's view to a verb: ability, duty, advice or how sure you are. They never take -s, and the verb after them stays in the base form. Start with the overview, then study the pairs that learners mix up most.
Lessons in learning order
Each lesson explains one rule and ends with a 20-question drill.
Modal verbs: can, could, must, should, may
No -s, no to, no do: what modals share, and which one fits each job.
Can vs could
Can for now, could for the past and for polite requests, plus the one past situation that refuses could.
Must vs have to: rules and obligations
Must is the obligation you feel or state. Have to is the obligation the world gives you. The two negatives mean completely different things.
Mustn't vs don't have to: ban or choice
Mustn't means you cannot do it. Don't have to means you can, but nobody requires it. The two negatives sit almost opposite each other.
Should vs ought to: advice and expectation
Should and ought to mean almost the same thing. Both give advice and state expectations; should is more common and takes questions and negatives without help.
Had better: strong advice
A fixed phrase for strong advice: had better + base verb, had better not for warnings, and no to after better.
May vs might: possibility and permission
May sounds a little more likely and a little more formal. Might sounds more remote and tentative. Both never take to.
Would rather vs prefer
Would rather + base verb for one choice now; prefer + noun or -ing for a general taste, and pref ... er ... to ... for comparisons.
Past modals of deduction: must have, might have, can't have
Must have shows certainty, might have shows possibility and can't have shows impossibility about the past. All three take have plus the past participle.