Question

定语从句的引导词由什么决定

Answer 1

由宾语从句的内容确定。
如果宾语从句为陈述句，则用that引导；
若宾语从句为一般疑问句，则用if/whether引导；
若宾语从句为特殊疑问句，则由构成特殊疑问句的疑问词引导。
简单的举三个例子：
1
He
said
(that)
he
was
a
student.
2
He
asked
if/whether
you
were
a
student.
3
He
asked
what
you
were.