1) You must have left your phone in the restaurant.
In this sentence, it is appropriate to use the modal verb must with a past reference to imply a degree of certainty, like the phrase in the original sentence almost sure. In this respect, the modal verb is used to express a high level of possibility.
2) You should have told me it was your birthday.
In this case, the modal verb should along with have told expresses an advice with a past reference. The modal verb should is always related to the function of giving advice.
A modal verbs are defined as the words that usually describe the possibility and necessity behind the words or phrases. For example, must, have etc.
A modal is generally refers to as the action in which the the verb is combined with the modal in order to make a tense.
You must have left your phone in the restaurant.
In this sentence, it is appropriate to use the modal verb must with a past tense have in order to provide better certainity to the sentence.
You should have told me it was your birthday.
In the above case, the modal verb should along with have told expresses an advice with a past reference.
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