Just to add one more point, based on my knowledge of common problems: remember that these verbs, known as modal verbs, are followed by bare infinitives,which means infinitives without "to". For example, you can only say :I CAN SWIM, not: I CAN TO SWIM. The exception is OUGHT TO which means the same as SHOULD: I OUGHT TO GO NOW.=I SHOULD GO NOW. Some of the modal verbs have their past forms, for example:I COULD SWIM is the past form of I CAN SWIM. But most don't have past forms and therefore are linked with "perfect infinitives": e.g. I SHOULD DO IT.( now or soon) I SHOULD HAVE DONE IT (yesterday) You can't say I SHOULD DID IT ,because "did" isn't an infinitive. There are 4 basic infinitive forms (+2 passive): DO e.g: She MAY DO it tomorrow. BE DOING e.g: She MAY BE DOING it now. HAVE DONE e.g: She MAY HAVE DONE IT yesterday. HAVE BEEN DOING e.g:She MAY HAVE BEEN DOING it when they called. Sometimes modal verbs have completely different past forms. For example: I MUST DO IT EVERY DAY. I HAD TO DO IT EVERY DAY when I was a child.

I should - suggestion, I could - I was able to I can - I am able, I have powers to do I may - it is allowed to me I must - strong form of obligation I shall - future form

Can Ability / Possibility Inability / Impossibility Asking for permission Request Could Asking for permission. Request Suggestion Future possibility Ability in the past May Asking for permission Future possibility Might Present possibility Future possibility Must Necessity / Obligation Prohibition Shall (More common in the UK than the US) Offer Suggestion Asking what to do Should Recommending action Uncertain prediction Will Instant decisions Offer Promise Certain prediction Would Request Making arrangements Invitation Preferences