Peter did lots of work变否定句,为什么?

Peter didn't do lots of work.
一般过去式的句子变否定句,要在实义动词前加didn't,然后再把过去式改为原形.

Peter didn't do lots of work.

Peter did not do lots of work,因为英语句子中只能出现一个时态，否定用了过去式，动词就要用原形了