1.如果前面一个句子有连词的话,动词一般用动词原形；
如：If she comes again,tell her that I am not at home.
2.如果前面一个句子没有连词的话,并且前面没有-ing分词或-ed分词的话,动词一般用-ing形式或动词不定式.
如：He stood there ,singing.(表示伴随）
He hurried to the station,only to find that the train had left.(意外的结果）
3.如果前面有-ing形式或-ed分词的话,动词要用动词原形.
如：Answering questions,speak aloud ,please.