在复合句中充当表语。
1. that在引导表语从句时无含义，不作成分，通常不省。
 1)表示事实、真理等的实际内容。
    The fact is that he is lying.
    The odds are that he will not do it.
 2)表示某人的意见、信念等的实际内容。
    Our belief is that things will improve.
    The trouble is that we are short of money.
2. whether引导表语从句（不可用if)
    His first question was whether holmes had arrived yet.
    The question is whether people will buy it.
3.连接代词引导的表语从句。
   This is what i want.
   The question is who can be chosen to be manager of the company.
   My question is which of them is better.
4.连接副词引导的表语从句。
   The problem seemed how we could make him understand it.
   The question is where we should go.
5. as if, as though引导的表语从句（常用虚拟语气）
   He looks as if he were angry.
   It looks as if it's going to rain.