赖世雄教你学英语语法
非谓语动词
2-9：分词结构——副词从句简化法
once,when, while, if, unless, though等六个连词所引导的副词从句中，若主语与主句中的主语相同时，也可以简化为分词结构，方法与单句简化法完全相同。
例：john, who is eager to learn english, has decided.
----if free, i'll go with you.
（如果有空，我将和你一起去。）
in those days, when he didn't know how to proceed in an emergency, he would consult his father.
----in those days, when not knowing how to proceed in an emergency, he would consult his father.
（在那段日子里，当他不知道如何处理紧急状况时，他都请教他父亲。）
he will do it if he is properly encouraged.
----he will do it if properly encouraged.
（如果适当地鼓励他，他会做它。）
unless you are otherwise instructed, you should do as i said.
----unless otherwise instructed, you should do as i said.
（除非另有指示，你应照我的话去做。）
while i was here, i had a good time.
----while here, i had a good time.
（在这里的时候，我很愉快。）
though he was seriously injured, he was not at all daunted.
----though seriously injured, he was not at all daunted.
（虽然受伤很重，他一点都不畏惧。）
once i am available, i'll let you know.
----once available, i'll let you know.
（一旦我有空，我会让你知道。）