可以的
It is the knife that made you out of trouble.
Whether that is possible does not have any needs to be judged by the manager.
I think that is a good idea.
The problem is what that costs.
The question whether that is a UFO has just been discussed.
I do not know whether that is dirty,but I made the city where that is everywhere.(注意后半句才含有
宾语补足语从句)
If that is nice,I will introduce that to my friend.
但只有
定语从句中的that才可能既做主语,又是引导词.