主句为一般将来时态，if从句用
一般现在时态：if 主将从现（这个比较好记）
since：（如果能够记住例句的话会比直接记语法更好用）
(1)接时间点：如 I have stayed in that country since 1995. 要求：谓v.用
现在完成时，而且须是延续性动词。
(2)接一段时间 + ago：如 They have lived in that house since two years ago. 要求：谓v.用现在完成时，而且须是延续性动词。
(3)接时间状语从句：如 I have loved her since I met her last time. / Lily has stayed at home since she left school. 要求：主句的谓v.用现在完成时，而且须是延续性动词；从句的谓v.用
一般过去时，而且须是终止性动词。
(4)用于句型“It’s / It has been + 一段时间 + since + 时间状语从句”中，译作“自从…以来, 已经…(时间)了”。如 It’s / has been three months since he moved to this city. 要求：主句的谓v.用一般现在时或现在完成时；从句的谓v.用一般过去时，而且须是终止性动词。
(5)用于句型“一段时间 + has passed since + 时间状语从句”中，译作“自从…以来, …
(时间)已经过去了”。如上个例句还可写成：Three months has passed since he moved to this city. 要求：主句的谓v.用现在完成时，而且动词必须是has passed；从句的谓v.用一般过去时，而且须是终止性动词。
(6)注：当since用于否定句时，主句的谓v.可以是终止性动词。如 He hasn’t seen the film since last month.
(7)相关词组：“从那时起”：ever since、since then、from then on.
注：ever since作为一个词组，放句尾可单独使用。如I have given up smoking ever since. 另外，还可在since后接句子。since前加ever，只是加重语气而已。如 I have given up smoking ever since I caught a bad cough.
(8) since还可引导原因状语从句，译作“由于；因为”，相当于because。如 Since he was lazy, he didn’t finish his homework.
while：当。。。。时
如果是名词，记住for a while，如果是连词，记住一般它后面跟的短语是be + doing
While用法小结
一、while作名词用，表示“一会儿”“一段时间”
1.She likes to lie down for a while after lunch. 她喜欢午饭后躺一会儿。
2.Coming to another country to study requires a big adjustment and it takes a while to fit in.
来到另一个国家求学，需要花一段时间来适应。 二、While作连词(1)引导时间状语
1.We must strike while the iron is hot.趁热打铁。
2.While she was listening to the radio，she fell asleep. 她听着
收音机睡着了。
(2)引起
让步状语从句，意思是“虽然…但是….”、“尽管…但是…”(多放于句首)1.While he loves his students，he is very strict with them..
虽然他爱他的学生，可是他对他们很严格。
2.While these experiments are interesting and useful，it is important to remember that they may not always tell us much.(BookIIIp125)
虽然这些实验很有趣也有用，但是很重要的是要记住，这些实验也可能不会告诉你很多。
(3)引起条件状语，意思是“只要”
1.While there is life there is hope.只要生命存在，就有希望。 2.While there is a way there is away.有志者事竟成。
三、While作并列连词用，意思为“而，然而”，表对比。而but表转折。注意用心比较体会while与but的句中用意
1.Some people waste food while others haven't enough. 有些人很费粮食，然而有些人却吃不饱。
2.The son was having a good meal at home，while the parents were working in the fields.
儿子在家吃好饭而父母却在田里辛勤劳作。
3.I ought to have helped her，but I never could. 我本该帮她但没能。
4.Honey is sweet，but the bee stings. 蜂蜜很甜，但蜜蜂有刺蜇人。 四.while，when，as的用法区别
都译为“当…的时候”，区别为：when后的从句
谓语动词既可是延续性动词，也可是短暂性动词，而且还可作并列连词，译为“这时”。但以下句型一定用when，作并列连词：
句型A：be about to do…when…正要做…这时(发生另一事) 句型B：was/were doing…when…正在…这时(发生另一事) 句型C：had done…when…刚要做…这时(发生另一事)
1.We were about to set out when it began to rain. 我们刚要出发天突然下雨了。
2.He had hardly sat down when his mobile rang. 他刚一坐下手机就响了
追问还有“等”呢