that到底怎么用?

一样的用.表示“是否”的用if后者whether.表示地点的用where.
还有：　连接代词主要有who,whom ,whose ,what ,whoever ,whomever ,whosever,whatever,whichever等.
　　连接代词一般指疑问,但what,whatever除了指疑问外,也可以指陈述.
连接副词主要有when,where,why,how,whenever,wherever,however等.
　　例句：　　He didn’t tell me when we should meet again.
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宾语从句that常可省略,但在以下情况下不能省略
　　A．当主句谓语动词带有两个或两个以上宾语从句时,可以省略第一个that,其他不能省略.
　　eg．I believe（that）you have done your best and that things will get bet?鄄ter．
　　B．当it作形式宾语时
　　eg．She made it clear that she had nothing to do with him．　　C．当宾语从句前置时