Ⅰ. because 是连词，后接句子， 是主句的直接原因，常用来回答由why 引导的特殊疑问句。如：
① Because he is ill, he is not here today. 因为他病了，所以今天没来。
② --Why are you in a hurry?
--Because I want to catch the first bus.
Ⅱ.because of 是复合介词，后接名词或代词，相当于名词词组。如：
He can’t come because of illness. 他生病了，所以没来。