一般表示被动,表示动作已完成的,用过去分词.如：
Seen from the top of the hill,the village is too small.
表示主动或者表示动作动作正在进行的,用现在分词.但是如果要强调非谓语动词的动作先于谓语动词发生时,可以用现在分词完成式.如：
Coming into the room, he found his father angry.
Having been hit by the big boy on the nose, the little boy began to cry.
如果用作目的状语或者表示将来的动作就用不定式,但是only+ to do特殊,是作结果状语.如：
I come here only to say good-bye to you.　目的状语
He searched the room only to find nothing. 结果状语

独立主格表示的是所填非谓语动词和句子主语没有任何关系（主谓关系或者动宾关系）,那么久在这个非谓语动词前面加上自己的逻辑主语,这就叫独立主格.如：
The test finished, we began our holiday.
Weather permitting, we are going to visit you tomorrow.