I could do nothing but let go.
There was nothing I could do but to let go.
请问这两句这样写文法正确吗?
如果是的话，为什么一句but后用原形，一句加to
谢谢

I could [do nothing] but [let go] 动词词组对应动应动词词组; There was [nothing I could do] but[to let go] 前面是名词词组 后面得对应能有同样功能的不定词(或动名词, probably)

非常感谢