当一个句子只有一个句号时,这是一句话,在一句话中一般必须要有主语,如果这句话比较长,需要两句话才能说完,那么另外一句就要当从句,或做状语.
I fell and hurt my leg, so I didn't go to school yesterday.
这句话前面半句是主句,后面半句是结果状语
because I fell and hurt my leg, I didn't go to school yesterday.
这句话后面半句是主句,前面半句是原因状语
所以because和so只能保留一个