A negative sentence contains a negative word.
否定句都包含一个否定词。
Though both sentences seem to express the same idea, but in different words, the first one is a negative sentence.
尽管两句话，表达的意思差别不大，但是用了不同的词汇。第一句话就是个负面的表达。
You cannot have a negative sentence after unless because that would make the sentence a double negative and senseless.
在unless之后不能接否定句，因为这就造成了双重否定，句子没有意义。
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