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I should have studied last night, but I didn't.

I've come across this sentence in a grammar book. Why the present perfect is used after "should", but the past simple is used to say "I didn't"? The book doesn't explain this.

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should is a modal verb. You use should with the base form of a verb to give advice or an opinion. e.g. You should go to bed. -> Going to bed is a good thing to do or the right thing to do. If you want to talk about the past you use the structure should + have + 3rd form of the verb. You use should with have to say that something was expected to happen in the past, although it has not in fact happened. e.g.: You should have helped us. Why didn't you? You should have done something = You didn't do it, but it would have been the right thing to do.

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You don't use "... I haven't" (present perfect) because you are talking about a finished action in the past. (last night) Therefore you use the past simple.

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  • But I'm also talking about a finished action in the past (last night) when I'm saying "I should have studied last night", right? By the way, it is supposed that you click "add a comment" link to answer to the comments) – Alexey Jan 20 '19 at 15:36
  • I know. I tried, but they didn't let me comment. Something about my reputation level… However, it's working now. Back to your question. Yes, you are talking about a finished action in the past, but modal verbs go with either the base form (infinitive) or with have + 3rd form. That is a fixed structure. – re_nez Jan 20 '19 at 15:41
  • Well, then how can we differentiate desirability (should-ness) of a particular past action from a desirability of current experience like we do hen we use the simple past and the present perfect? – Alexey Jan 20 '19 at 15:57
  • Modals are always used with other verbs. They are a special kind of auxiliary verb and behave differently (compared to main verbs). Modals do not usually show whether you are talking about past, present or future. Usually you indicate this in other ways. For example, by putting an auxiliary verb and a participle after the modal. e.g. 'should have studied'. – re_nez Jan 20 '19 at 16:24
  • You use the structure: modal verb + auxiliary verb + participle, to say that either you expect something to have happened already or something was expected to happen. e.g. You should have heard by now, that I'm fine. -> Here you are saying that you expect that something has happened already. vs. I should have studied last night. -> Here you are saying that something was expected to happen, but didn't. – re_nez Jan 20 '19 at 16:25

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