Which sentence is true grammatically? Would you please explain me?
Sara has a ring in her left hand. she must have gotten married.
Sara has a ring in her left hand. she must get married.
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Sentence number 1 is correct. It implies that the last time you saw her she did not have a ring and now she does, so you are inferring that in the mean time she got married. "Must" here describes your logical process of coming to that conclusion.
Sentence number 2 is grammatically fine but makes no sense because "she must get married" means she is not yet married but for some reason she absolutely needs to in the immediate future. It doesn't say she is about to get married (i.e., is engaged). In the second case "must" no longer refers to your inferrence but now intensifies her need to get married immediately--very odd since marriage is normally mutually consensual.