呵呵,你的理解没有错。书上语法也没有错。must和need在必须意义上是一对。即肯定句用must do,否定句疑问句用need...do? needn't do...当然,must也可以直接提前构成疑问。但否定则必须使用needn't do了。此时need无三单和过去式变化。
I must be home before ten o'clock.
Must i be home before ten o'clock? yes,you must. no,you needn't.
Need i be home before ten o'clock? yes,you must. no,you needn't.
当need用于肯定句时,常可解释为需要,而不是必须。此时成为普通的实义动词。拥有三单和过去式变化了。但是不能再提前构成疑问,也不能+not构成否定了。要借助do了。
I need a book. Do you need a book ? yes,I do. No,i don't.
I need to have a rest. Do you need to have a rest ? Yes, i do. No, i don't.
your sock needs mending. Does your sock need mending ?Yes,it does. No,it doesn't.
He needed a book last night. Did he need a book last night ?yes,he did. No,he didn't.
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