当作及物动词(vt)时：
explain可以接to和从句.比如：explain this rule to me (explain sth to sb)
Can you explain how the machine operates?
He explained that he had been cheated.
作为不及物动词(vi)时：explain后面要加介词:
I've got to explain about(介词) it.我得解释一下这件事.
一般都是用explain sth to sb,explain + 从句,explain about sth.