请讲解句中When用法

The planes had been planting seed for nearly a mon掸範侧既乇焕岔唯唱沥th when it began to rain.---新2第62课的句子. 句中为什么要用when?为神马不写成 before it began to rain? when常作conj,若理解成当...的时候,句子意思不通顺.求解释?

1.引导特殊疑问句，可译为“什么时候”
When do you think the patient can be operated on？你认为应该什么时候给病人做手术？
2.可引导不定式短语做主语或宾语
Ask him when to open the red wine.问问他什么时候打开红酒.
When to put the new school to use will be announced tonight.新学校什么时候投入使用将在今晚宣布.
3.可引导限定性和非限定性定语从句，修饰主句中表时间的先行词，并在从句中做状掸範侧既乇焕岔唯唱沥语。
There will be one day when the impossible today may become possible.(限定性定语从句)将来会有一天今天不可能的也许会成为可能。
We will put off the picnic until next week，when the weather maybe better.(非限定性定语从句)我们将把野餐推迟到下星期，那时候天气可能会好一些。
4.引导名词性从句
1)引导主语从句
When the foreign friends will arrive is not known yet.外国朋友什么时候到还不知道。
2)引导宾语从句
Lily is asking when she can go back to her hometown again.莉莉问她什么时候可以再回家乡。
3)引导表语从句
That was when I was thirteen.那是我十三岁时候的事了。
4)引导同位语从句
Can you answer the question when Penicillin was discovered？你能回答这和问题吗———盘尼西林是什么时候被发现的？
注意：when引导的主语从句有时会用形式主语it代替When the foreign friends will arrive is not known yet.=It is not known yet when the foreign friends will arrive.
5.引导时间状语从句，可译为“当…时候”，本文开始课文句子中的when即为此用法。
When he got up，he felt dizzy.当起身时他感到头晕。
When he looked back，the woman had disappeared.当他回头看时，那个女人已消失踪迹了。
6.when引导从句的特例：
1)表示突然发生某事或紧接着发生某事，常可译为“…还没/刚刚…就…”，“…正在…忽然…”或“正要…这时…”，主要用于“had done…when…，be doing…when…和be about to do…when…”这三个句式
I had hardly opened the door when the man hit me.我刚打开门，那个人就打了我一拳。
I was reading a book attentively when an old friend came to see me.我正在全神贯注地读一本书，忽然一位老友来访。
I was about to swim in the river when the guide told me there was man-eating fish in it.我正要下河游泳这时导游告诉我河里有食人鱼。
2)表示对比，常可译为“本该/本可以…却…”
Why are you here when you should be at school？你本应在上学，怎么会在这儿？
Why did you take a taxi when you could walk here within five minutes？你本来五分钟就可以走到这儿，为什么还打的？
3)表示让步，常可译为“既然”或“在…情况下”
How can they learn anything when they spend all their time watching TV？既然他们把所有的时间都花在了看电视上，又怎么会学到东西呢？

when引导词 it began to rain为时间状语的定语从句,修饰主句的动词 planting for nearly a month ,The planes had been （when it began to rain） planting seed (中文式) 过去完成进行时

一、when：当主句中的先行词(即主句中被后面定语从句修饰的词)是表示时间意义的名词时，它只能作定语从句的时间状语，放在定语从句句首。如果定语从句的引导词是作该定语从句的主语或宾语，则要改用关系代词that或which来引导。例如： 　　the days when we used foreign oil are gone. 我们用洋油的日子一去不复返了。 　　i'll never forget the day when i was born. (=i'll never forget my birthday.) 我永远不会忘记我出生的日子。 　　it happened in november when the weather was wet and cold. 这事发生在天气又湿又冷的十一月。 　　in the years that (which) followed, marx kept on studying english and using it. 在这之后的几年中，马克思继续学习和使用英语。(that作定语从句"that followed"的主语) 　　the day (that) i always remember in all my life is my birthday. 我一生中最难忘的日子是我的生日。(that作定语从句"that i always remember in all my life"的宾语，that可以省略)