as引导的是比较分句，that引导的是关系分句。在表示同一个这个意义时，既可以用as分句，也可以用that分句。但需要注意用that分句时，只有部分先行词可以用关系副词that引导，所以如果关系词作状语，能否用that需要慎重。

I studied at Cambridge at the same time as he did. 先行词为time, 可以用关系副词that引导关系分句。

If you’re likely to be travelling on the same train as I am, I’ll look out for you. 用that引导关系分句时，需要加介词on. 即 on the same train that I am on.

He works in the same shop as I do. 本句不可以用that引导。除非改为 in the same shop that I work in.