1) You must have left your phone in the restaurant.
In this sentence, it is appropriate to use the modal verb <em>must</em> with a past reference to imply a <em>degree of certainty</em>, like the phrase in the original sentence<em> almost sure.</em> 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 <em>should</em> along with <em>have told </em>expresses an advice with a past reference. The modal verb <em>should</em> is always related to the function of giving advice.