i must water the garden first.do

问题描述:

i must water the garden first.do
i must water the garden first.do you have to water it now?第一句为什么可以不加have to .第二句要加

第一句把 must 换成了 have to 没有问题:I have to water the garden first.
第二句把 have to 换成了 must,这个句子就有语法错误了:Do you must water it now?
疑问助动词 do 和情态动词不能同时出现在一个句子里.你再看看上面这个句子有什么问题?