I don't know when I can post the letter.改为同义句I don't know ___ ____英语解答

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?

人气：144 ℃　时间：2019-12-13 17:18:24

解答

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