all she does is cry←省略了to?为什么可以省略?

问题描述:

all she does is cry←省略了to?为什么可以省略?

All she does is cry←省略了to?为什么可以省略?
此句的句子是这样分析:
因为句中有does/do/did时,可以省略不定式符号to.
She did nothing last night but cry.