It's crucial that I study for it because it makes up 30% of the final exam.

问题描述:

It's crucial that I study for it because it makes up 30% of the final exam.

It's crucial (that I study for it 【because it makes up 30% of the final exam】).
总共是2个从句
()内的是主语从句
【】内的是原因状语从句