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First of all, let me give you these example sentences:

  • there is a storm
  • there was a storm
  • there has been a storm
  • there had been a storm

Every one of the above examples expresses that there certainly is/was/... a storm.

Now I would like to express that I think that there maybe is/was/... a storm, for example because I saw all the fallen trees. In the present tense it is very clear: there must be a storm.

If I want to say the same in the past perfect the word must won't change: there must have been a storm.

What I do not understand is what type of word must is this example is. Why doesn't it change? If I say I must go to school it changes to I had to go to school in past perfect because must is a Modal Auxiliary then.

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First, note that there must had been a storm is not grammatical in most varieties of English. There must have been a storm is what I think you meant.

Secondly, the answer is that must is a modal verb, but like could it hasn't got a past tense. (Historically, they are both past tenses, and in some senses could still functions as the past of can. Must is isolated, with no corresponding present-tense verb).

If you want to express obligation in the past, must is not available, so you need to substitute another expression, typically have to (which means almost the same as must, and has a past tense: had to).

  • Thank you so far! But is there a reason why must had been is not grammatical? And how would the sentence look like if we used present tense for the word must? There had to ... a storm? – Elastic Lamb Mar 13 '18 at 21:07
  • @ElasticLamb. Because modals, without exception, are followed by the base form of the verb (sometimes called the infinitive), not by a tensed from such as had. The verb in question can be another auxiliary, eg must have been, must be seen. I don't understand the second question in your comment. There must be a storm. There has to be a storm. There must have been a storm. There had to be a storm. – Colin Fine Mar 13 '18 at 23:53
  • Alright, that I have understood. But what exactly means have been then? I thought it would be present perfect. Sorry for my weird questions! – Elastic Lamb Mar 14 '18 at 7:09
  • Ah, @ElasticLamb, this gets a bit complicated. With non finite forms, have with the past participle is past, but not necessarily perfect, in meaning. So there must have been a storm (which only makes sense with the epistemic meaning of must ) means something like "I conclude that there was/has been a storm". – Colin Fine Mar 14 '18 at 18:31

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