I don't know what i can do to help her.（改为同义句） I don't--- --- ---to help her.

I don't know what i can do to help her.（改为同义句） I don't--- --- ---to help her.
I don't know what i can do to help her.（改为同义句） I don't--- --- ---to help her.

I don't know what i can do to help her.（改为同义句） I don't--- --- ---to help her.
应该是四空吧,楼主是不是打错了?
I don't know what I can do to help her.
= I don't (know what to do) to help her.

know how

know ,how,