He is writing a letter in his study. (否定)

问题描述:

He is writing a letter in his study. (否定)

He isn't writing a letter in his study.
变否定
有be动词的句子在be动词后加not.
有情态动词的句子在情态动词后加not.
没有be动词的句子在动词前加don't(主语非单三主语)或 doesn't(主语是单三主语)
现在完成时在have,has后加not没有be动词的句子在动词前加don't(主语非单三主语)或 doesn't(主语是单三主语)这句话里,是三单吧,你是不是打错了,打成了单三都一个意思