分词作状语的五种情况：
一、表时间，可以和时间状语从句互换。如：
Studying in the university, he was very popular among his classmates.=When he studied in the university, he was very popular among his classmates.
二、表原因，可以和原因状语从句互换。如：
Not knowing her address, I wasn’t able to contact her. =Because I did not know her address, I wasn’t able to contact her.
三、表条件，可以和条件状语从句互换。如：
Walking to work, you will save quite a lot of money.=If you walk to work, you will save quite a lot of money.
四、表让步，可以和让步状语从句互换。如：
Defeated, he remained a popular boxer. =Although he was defeated, he remained a popular boxer.
五、表方式，如：
He earns a living driving a truck.=I’m returning you letter as requested.
现在分词在句中作原因状语：
分词短语在句中作原因状语时，相当于一个原因状语从句。与时间状语一样，也要注意分词所表示的动作与谓语动作的先后关系。
当分词所表示的动作与谓语动作同时发生或几乎同时发生时，用分词的一般形式。此时分词的逻辑主语须是句中的主语。这样的原因状语可以换成because, as引导的原因状语，该从句谓语动词用一般过去时。