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The topic is what the sentence or the paragraph is about while the main idea is what the writer is trying to convey in his entire message.Topics are simpler and only use a word or a phrase; the main idea is stated as an entire sentence.
3.The topic must not be specific nor should it be general while the main idea must be complete.
4.The main idea may appear in the beginning, the middle, or the last part of the paragraph while the topic can be found in the sentence or in a paragraph