摘要：含有情态动词的句子改为否定句或疑问句不必加助动词.只需要将情态动词置于主语前.或在情态动词后加上not. 例如:Must I write a letter to cheer him up? You needn’t wait for me. 情态动词的过去式.例如can-could.may-might.但是少数情态动词只有过去式.例如used to表示过去常常.must的过去式往往用had to代替. 情态动词后还可接不定式的进行式.完成式和被动式. 例如:He can’t be doing his homework now. 他现在不可能在做功课. He must have attended the meeting last night. 昨晚他一定参加了会议.

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