The sentence 'I can't believe it!' is exclamatory. An exclamatory sentence typically if not always ends in an exclamation mark and expresses strong emotion (disbelief, in this case). An exclamatory sentence differs from a declarative sentence in that it serves the purpose of conveying the speaker's feelings, rather than stating a fact or opinion. An imperative sentence is a command, while an interrogative sentence is a question, ending in a question mark.
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