HAD TO - past form of must SHOULD/OUGHT TO HAVE I should have + verb of thinking (thought, realized, known) - eg. I should have known this company is a scam. Criticism of someone's action - You should have told her. Expectation - It should have been cheaper. Polite expression when recieving a gift - You shouldn't have. COULD - past ability COULD HAVE/COULDN'T HAVE - past possibility/uncertainty, unwillingness, It couldn't have been + adjective = superlative MAY HAVE/MIGHT HAVE - action, that didn't actually come true, uncertainty, ironical presumption: I might have known + would WOULD NOT - unwillingness WOULD HAVE - events that didn't happen NEEDN'T HAVE/DON'T NEED TO - needn't have- unnecessary action which has already been accomplished Don't need to - You know it is unnecessary, so you avoid it MUST HAVE/CAN'T - absolute certainty