used to do的否定used to do正式的否定形式是used not to do,口语中可以说didn't u

英语中理论上有12个特殊动词和它们的变化形式在构成否定形式时是直接在后面加 not, 它们是
be (am, is, are, was, were)
have (has, had) 注：英国只有作“有”解释及助动词时,美国只在作助动词时
can (could)
may (might)
must
shall (should)
will (would)
do (does, did) 只作助动词时
dare
need
ought to
used to
其它动词都要用助动词do, does, did 来构成否定,这是他们的习惯,只能遵守.不过现代英语越来越多把 dare 和 need 用作普通动词,用do, does, did构成否定.ought to 也常常用should 来代替.used to 在口语中也说成 didn't use to.
同样,疑问形式也是如此.

一般都是这样。是to do的在to前加。没有to就在动词后加not