What is the difference between replace and substitution? In Chinese both of them mean 替换.

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Replace is a verb whereas substitution is a noun. Perhaps you want to compare replace with substitute or replacement with substitution.

They are synonyms and practically the same in meaning. That said, replace seems more permanent than substitute. For example, if you said a teacher was replaced, it means the old teacher has retired or moved on. If you said a substitute teacher was teaching, I would assume that the original teacher will return at some point.