Modals
What are the differences in use between must have, can’t have, should have and needn’t have?
must have
can't have
should have
needn't have
must have
When we use must have and the past participle (must have phoned), we are making a deduction about something that has happened. We are saying, ‘I feel sure that this is the case’:
If your keys aren’t in your pocket, you must have left them in the house.
can't have
We use can’t have and the past participle (can't have phoned) when we are making a deduction about something that didn’t happen in the past. That is, you believe, based on present evidence, that something didn’t happen (or hasn’t happened):
He’s not looking. He can’t have heard you. Shout again!
should have
Should have and the past participle (should have phoned) expresses the idea that something was desirable or needed, but did not take place:
I should have phoned my mum last night, but I forgot. (I needed to phone my Mum but I didn't phone her)
Shouldn’t have and the past participle (shouldn't have phoned) expresses the idea that something did take place but that it wasn’t desirable or needed:
You shouldn’t have eaten so much chocolate, you’ll be sick. (you ate a lot of chocolate and it wasn't a good idea)
needn't have
Needn’t have and the past participle (needn't have phoned) is opposite in meaning to should have. It indicates that something was done, but that it was not necessary. That is, the person who did something thought it was necessary:
A: You needn’t have cooked a meal this evening. I had lunch at The Chinese Dragon with Tom.
B: Well, why didn’t you phone and let me know?
Back to top
| 
