I don't know when I can post the letter.改为同义句I don't know ___ ___ ___the letter.请问这是什么句型?为什么这么填?

问题描述:

I don't know when I can post the letter.改为同义句I don't know ___ ___ ___the letter.
请问这是什么句型?为什么这么填?

when to post
这是正解

when to post
我不知道什么时候把信寄出。

when to post
这是疑问词+to do sth的省略疑问句形式
I don't know what to do next=I don't know what I will do next