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  1. The children 'must have been playing' in the park since we gone shopping please find them out it's getting dark.
  2. Just look at that car ! His father 'must have bought' it for him because he has been sad since her mother died.

My question is do modals have the ability to change the tense of the sentences? Like 1 is in present perfect continuous and 2 is in present perfect.

They say we use modal+have+been for past events. We use present perfect also for past events but it has relevance at the time of speaking. I need in depth answer please if anyone can provide it.

  • An "in depth" answer would be very long. Have you tried searching the internet? For example, this article should have all the depth you need. If it's not sufficient, please add more detail to your answer to explain what you still don't understand. – Andrew Jun 10 at 19:01
  • Must have been playing versus must be playing. Must have bought versus must buy. It's not the modal that's defining the tense here. Unless I'm misunderstanding your question. – Jason Bassford Supports Monica Jun 10 at 19:12
  • No I searched the internet they say like this- These past (have+ past participle)modal verbs are all used hypothetically, to talk about things that didn't really happen in the past. – Rocky Jun 10 at 19:20
  • I know what(modals) they mean I'm asking like ' have been playing' is present perfect continuous and 'must have been playing' here 'must' just adding the certainty to the sentence also 'have bought' is present perfect and 'must have bought' here 'must' just adding certainty only without changing the tense? – Rocky Jun 10 at 19:30
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Most (maybe all) modals have two meanings, a deontic one about the real world (including social things like obligations), and an epistemic one about somebody's knowledge or expectation of things.

So I must go is deontic: I am stating that there is an obligation on me to go. But He must be on his way is epistemic: there is no obligation on him, the speaker has deduced, or is otherwise sure, that he is on his way.

When a modal is used with have, the meaning is very often epistemic: He must have gone cannot be deontic (we have to express the deontic meaning in other ways, such as "He had to go", or "He was obliged to go".

Other modals can take either meaning in the perfect, depending on context: I should have seen it is deontic, but I think he should have got there now is almost certainly epistemic.

So your examples are both epistemic "must".

But in answer to your main question, modals with "have" behave like past tenses, but not necessarily like (present or past) perfects: there is not necessarily any present relevance. "He should have seen it" can correspond to either "he saw it" or "he has seen it".

(Note also that epistemic will shows that will is indeed a modal, not a marker of "future tense": He will have seen it by now is epistemic and explicitly not future: it means something like "I conclude that he has seen it by now").

  • So according to your explanation the sentences should be like this:-1.The children 'must be' in the park since we gone shopping please find them out it's getting dark. 2.Just look at that car ! His father 'must buy' it for him because he has been sad since her mother died. – Rocky Jun 11 at 4:38
  • It's really confusing now because if I say modal+have+past participal sentences what would they mean really? Like you said "He should have seen it" can correspond to either "he saw it" or "he has seen it". – Rocky Jun 11 at 9:17
  • @Rocky, "He should have seen it" could mean "It was his duty/responsibility to see it" (deontic) or "I expect that he has seen it" (epistemic perfect) or "I expect that he saw it") (epistemic non-perfect). All are possible. – Colin Fine Jun 11 at 16:29
  • Sir you mean that in case of 'modal with have' we can only judge what the sentence mean by only in what context it is interpreted by the speaker present or in past is depend on situation. Like "the children 'must be' in the park since we gone shopping please find them out it's getting dark". This is the present context and sentence like "the children 'must be' in the park" this may be in the past context. – Rocky Jun 11 at 17:12
  • No, @Rocky. The present/past distinction is the one which is required, but for many modals it is made not by using a past form of the verb, but by using the have construction. "The children must be in the park" is present, and it is not idiomatic to use present verbs with since. So "The children must have been in the park since we went shopping". – Colin Fine Jun 11 at 19:44

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