就是because+句子=because of+短语的意思是因为+基本句子=由于；因为+短语 .
because后面接句子；because of后面接非句子,即短语或者非谓语.
如：1.I did not answer the telephone at that time,because I was washing my clothes then.(去掉because,后半部分仍然是个句子）
2.I did not see you because of his arrival.(去掉because of,后半部分不是句子）