It was evening ______ we reached the little town of Winchest

It was evening ______ we reached the little town of Winchester.when为什么不可以
什么时候用when 什么时候用before

都可以吧 只是意思不同 其实这2个词 填的时候注意语境 意思对就可以 经常2者可以互换的、
when
意为:我们到Winchester小镇的时候是晚上.
before
意为:我们还没到Winchester小镇就到晚上了.
一． Before
before作从属连词,引导时间状语从句,表示“在……之前”.从句表达的是一已知或可以预见的事件,通过此事件的时间来告知未知的、主句事件的时间.如：
1.The bus had left before I arrived.我到达之前汽车已经开走.
2.She changed her hairstyle in her hometown before she came to Changing for a better job.在到重庆来找个好些的工作之前,她在家乡改了发式.
3.Make sure to turn off all the lights before you leave the room.离开房间前确保关掉所有的灯.
4.We had not gone far before we saw that hotel.在看见那旅馆之前我们没走多远/我们没走多远就看见了那旅馆.（before有时译作“就”.这里“看见旅馆”是已知事件.）
5.You must finish your homework before you go out to play basketball.你在出去打篮球之前必须完成作业/你必须完成作业才能出去打篮球.（before有时译作“才”.这里“出去打篮球”是已有准备的、可以预见的事.）
6.The meat dropped from his mouth into the river before he knew it.在他知道之前,那块肉已从他嘴里掉到河里/他还没弄清怎么回事,那块肉已从他嘴里掉到河里.（he knew it是可以预见到的、迟早会发生的事.）
二． When
1.when作从属连词,引导时间状语从句,表示“在……时”.从句可以位于主句之前或之后,表达的是一已知的或可以预见的事件,通过此事件的时间来告知未知的、主句事件的时间.如：
1.You were out when I dropped in at your house.我到你家拜访时你不在家.
2.What were you doing when I phoned you last night?昨晚我给你打电话时你在做什么?
3.The discussion came alive when an interesting topic was brought in.当说到一个大家感兴趣的话题时,讨论立刻活跃起来.
4.---Can I drive on the free way,Mr.Green?
---You can when you get a bit more skilled.
——格林先生,我可以在高速公路上开车吗?
——到他技术再熟练些你就可以了.
2.when作并列连词,在前一分句后引出一个新的突发事件,前一分句描述新事件发生的背景,为新事件作铺垫.如：
1.We were swimming in the lake when suddenly the storm started.我们正在湖里游泳,突然风暴骤起.（前一分句常用过去进行时描述背景.“风暴骤起”是原本没有预见到的突发事件.）
2.He was walking on the street when three men jumped up and stopped him.他正在街上走着,突然三个人跳到他跟前拦住他.
3.He was about to tell me the secret when someone patted him on the shoulder.他正要告诉我秘密突然有个人拍拍他肩膀.（前一分句也常用be about to do sth.描述背景）
4.I had just locked the door when I realized I had left my keys on the kitchen table.我刚锁上门突然意识到我把钥匙忘在厨房的案板上.（前一分句也可用过去完成时描述背景）