由从属连词that，whether引导 the question is whether you should ask them for help. 问题是你是否需要向他们求助。 the trouble is that i have never done the work before. 麻烦的是以前我从没做过这个工作。由连接代词what，who，which引导 beijing is not what it used to be.
北京不再是过去的北京了。 what she wants to know is which dress she should buy. 她想知道的是应该买哪件衣服。
