“When I was younger, I wasn’t a great student and looking back, I think that I should have worked a bit harder at school. If I had worked harder I might have got better results. In reality, I got decent results in my exams, so I can’t have been a terrible student and must have understood the material quite well!”
Perhaps you recognise some of the verbs in that quote. “Should”, “must” “can’t” and “might” are all modal verbs but are used here to speculate about the past.
If I say “I should study” I am talking about what I think would be a good idea now, as we use “should” to make recommendations or describes what we are supposed to do.
If I say that “I should have done something” I am talking about what would have been a good idea for me to do or what I was supposed to have done in the past.
How to Use Past Modals.
This is how we use modal verbs to speculate or talk about the past, we always combine;
Modal Verb + Have + Past Participle
There are various modal verbs that we use in this way, we can use “Should / ought to”, “Must” “May / Might / Could” & “Can’t”.
|When you are sure of something.||When you aren’t sure of something (speculation).||When you are sure that something is impossible.||When you are making recommendations about the past.|
|Must + have + participle||May / Might / Could + have||Can’t + have + participle||Should / ought to + have+ participle.|
|“Tom’s car isn’t here, he must have gone out.”||“He might have gone to the supermarket.”||“He can’t have gone to work, it’s Saturday.”||“He should have told me if he was going out!”|
Examples of Use in Context.
So, if I am in my apartment and can’t find my keys, I can logically say that ” I must have had my keys when I came home or I wouldn’t have been able to open the door!” I can be quite certain that I had my keys, or it would have ben impossible to get into the flat.
If I am thinking about where I left my keys, I can speculate by saying “I may have left my keys in my jacket pocket, or I might have left them on the table by the door, or I could have left them in my trouser pocket.”
If I am sure that it is impossible that I lost my keys outside the apartment I can say “I can’t have lost my keys at the pub because I used them to open the door when I came home last night.”
I could also tell myself what I should have done (what would have been a good idea) by saying “I should have put my keys in the bowl by the door, then I wouldn’t have this problem now!”
Test yourself with the quizzes below, each one practices a different aspect of the grammar.