When he will arrive is not known.

I don't know how I can get through the exam.

3．连接副词有when, where, why, how 等，它们既起连接作用，本身又作从句的状语。

how many(much, far, long, often, soon)等引导主语、宾语、表语从句

2.连接代词有who, whom, whose, what, which, whatever, whoever, whichever等。它们在从句中起连接作用，本身又作从句的主语、宾语、宾补或定语。

1.从属连词有that, whether, if。它们只起连接作用，不充当从句的任何成分