In a sentence, "rat" can be either the subject or the predicate depending on its role within the sentence.
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As a subject, "rat" would be the noun that the sentence is about. For example: "The rat is in the garden." Here, "the rat" is the subject.
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As part of the predicate, "rat" could be included in a verb phrase or description that tells something about the subject. For example: "He is a rat." In this case, "rat" is part of the predicate, describing what "he" is.
So, it depends on how "rat" is used in the context of the sentence.