经常不知道名词性从句什么时候用whether和if，什么时候用that？

Question

例如这道题 I am not sure whether/if he can earn a living. 为什么不能用that？ 求助各位大神！

Answer 1

1.主句从句只能用whether
whether
he
will
come
isn't
clear.
2.
表语从句只用whether
The
problem
is
whether
he
will
come.
3.
及物动词后的宾语从句whether和if可以换用。
I
don't
know
whether/if
he
will
come.
介词后宾语从句不用if