could / couldn’t – past possibility / opportunity / risk
might / might not – past potential
needn’t – not necessary or not required in the past
Criticising Why did you do it on your own? You could have asked me for help. You were late to work. You shouldn’t have gone out last night. I can’t believe you drove that fast! You might have gotten caught. should / shouldn’t could / couldn’t might / might not needn’t You needn’t have come to the meeting. It was just for the sales team.
Speculating He looks so tired. He mustn’t have had much sleep last night. I’ve not seen them together for a long time. They might have separated. I saw him in KLCC, so he can’t have been at the meeting. Ng finished the exam in just 20 minutes. He couldn’t have answered all the questions.