_______the fight stopped,travel across the country has been quite safe.A.since B.When C.After D.UnlessABC中好难选呀!能给我详细说说这3个词怎样用吗?把中文套上去好像都可以.

_______the fight stopped,travel across the country has been quite safe.A.since B.When C.After D.UnlessABC中好难选呀!能给我详细说说这3个词怎样用吗?把中文套上去好像都可以.
_______the fight stopped,travel across the country has been quite safe.
A.since B.When C.After D.Unless
ABC中好难选呀!能给我详细说说这3个词怎样用吗?把中文套上去好像都可以.

_______the fight stopped,travel across the country has been quite safe.A.since B.When C.After D.UnlessABC中好难选呀!能给我详细说说这3个词怎样用吗?把中文套上去好像都可以.
A,意思是自从那个航班停飞了,整个城市都变得很安全了
顺治皇帝从见到她那天起就再也没有认真当官：The Emperor Shunzhi had never taken his duty seriously since he saw her that day.
说的是过去的事情,而且是过去已经完成了的事情.
when 和while的用法区别
两者的区别如下：
①when是at or during the time that,既指时间点,也可指一段时间；
while是during the time that,只指一段时间,因此when引导的时间状语从句中的动词可以是终止性动词,也可以是延续性动词,而while从句中的动词必须是延续性动词.
②when 说明从句的动作和主句的动作可以是同时,也可以是先后发生；while 则强调主句的动作在从句动作的发生的过程中或主从句两个动作同时发生.
③由when引导的时间状语从句,主句用过去进行时,从句应用一般过去时；如果从句和主句的动作同时发生,两句都用过去进行时的时候,多用while引导,如：
a.When the teacher came in,we were talking.
当此句改变主从句的位置时,则为：
While we were talking,the teacher came in.
b.They were singing while we were dancing.
④when和while 还可作并列连词.when表示“在那时”；while表示“而,却”,表对照关系.如：
a.The children were running to move the bag of rice when they heard the sound of a motor bike.
孩子们正要跑过去搬开那袋米,这时他们听到了摩托车的声音.
b.He is strong while his brother is weak.
他长得很结实,而他弟弟却很瘦弱.
表达“在……之后”的词
1.after prep.,adv.conj.
(a)After testing,the engine was put into immediate service.
经过试验以后,发动机立即交付使用.
(b)It happened twenty years after.
这事发生在20年之后.
(c)After one cycle is completed,another cycle begins.
一个循环完成后,另一个循环又开始.
2.in prep.
(a)In about five years,what I have said today will become true.
大约5年以后,我今天所说的就会变成现实.
(b)In the four years that followed,I studied in the Department of Accounting.
在以后的4年中,我在会计系学习.
(C)It's two o’ clock.I'll come in an hour.
现在是两点,我一小时后到.
〔注〕：在表示以现在为起点的“在……之后”这个含义时,介词in之后应接时间段(a period of time),介词after之后则应该接时间点(a point of time).比较：
(a)I've ordered a taxi.It will arrive here in 10 minutes.
我已经叫了出租车,10分钟以后就会到.
(b)He will graduate in four years.
他4年以后毕业.
(c)He will be back after three o'clock.
他3点以后到.
但是,当以某个过去的时间为起点时,after之后既可以接时间点,也可以接时间段.比如：
(a)Just after six the meeting was over.
刚过6点,会议就结束了.
(b)After half month of war Japan was badly defeated.
经过半个月的战争,日本被打得惨败.
(c)After seven days of this I went to a party.
这样过了7天以后,我去参加一个聚会.
3.later
(a)Fuller report will be published at a later date.
更详细的报告会在以后出版.
(b)I don't know what would happen later on.
至于以后怎么样,我就不知道了.
(c)He handed in his resignation,and later (on)regretted his hasty action.
他交了辞职书以后,又为自己的草率感到后悔.
〔注〕：later还可译为“过一会儿”,“过了一会儿”,“后来”等.
4.afterward(s) adv.
(a)She had her supper and went bed soon afterwards.
她吃过晚饭之后不久就去睡觉了.
(b)I didn't see him afterwards.
以后我再也没有见到他了.
(c)I was a bit rude to her at the meeting,and afterwards I apologized.
在会上我对她是有点粗暴,但是（散会）以后我向她道了歉.
Unless:conj.除非,如果不
unless是从属连词,引导条件状语从句,含有否定意义,相当于if条件状语从句的否定形式.所以unless = if not.注意：unless引导的条件状语从句和if条件状语从句,与其它时间状语从句一样,用一般现在时代替将来时.
1.Unless you take more care,you’ll have an accident.
(如果不多加小心的话,你会出事故的.)
2.My baby sister never cries unless she is hungry.
(我那刚出生的妹妹除非饿了,否则她是从来不哭的.)
3.Unless bad weather stops me,I jog every day.
(除非坏天气阻拦我,否则每天我都慢跑的.)