The planes had been planting seed for nearly a month whtn it

The planes had been planting seed for nearly a month whtn it began to rain.这句话中的when是什么
用法,为什么用when引导而不用before.

when引导时间状语从句
　　when是引导时间状语从句最常用的一个词,它涵盖的时间范围比较广泛,在作“当……的时候”解时,有下面几种常见用法：
　　1.表示在某个具体的时间里发生的动作.此时,when不能用as或while替换.
　　When she comes,I shall tell her to wait for you.她来的时候我会告诉她等你的.(指具体的时间点)
　　When his father died of illness in 1980,he was only two years old.1980年他父亲病逝时他只有两岁.(指1980年这个具体的时间点)
　　2.表示一个动作与另一个动作在同一时间发生,或一个动作在另一个动作所延续的时间范围之内发生.此时,when可以用as或while替换.
　　I hurt my shoulder when (while,as) I was playing basketball.我打篮球时弄伤了肩膀.
　　He entered the room when (while,as) the meeting was going on.正当开会的时候他走进了房间.
　　When (While,As) Tom was reading,Jim was writing.汤姆看书的时候,吉姆在写东西.
　　两个同时发生的动作,如果不是一次性而是重复性的,即过去或现在的习惯动作,可用连词when或whenever,意思是“每当……的时候”.例如：
　　When (Whenever) he goes to town,he always visits his aunt.每当他去镇上,他都去拜访他姑姑.
　　She always felt ill when (whenever) she ate oyster.她每次吃牡蛎都会生病.
　　在上述两例中,不管是用when还是用whenever,主句动词和从句动词或者都用一般现在时,或者都用一般过去时,分别表示现在或过去的习惯动作.
　　3.表示从句的动作发生后,主句的动作立即发生.此时,when表示动作的先后顺序,相当于after,不能用as或while替换.
　　When he had done his homework,he watched television.=After he had done his homework,he watched television.他做完作业后看电视.
　　When he had painted the windows,he painted the doors.=After he had painted the windows,he painted the doors.他把窗户油漆完后,又把门油漆了.
　　在这里,when从句的谓语动词都用了完成体,when从句既可以置于句首,也可以置于句尾.但是,如果when从句的谓语动词用一般现在时或一般过去时,when从句只能位于句首,表示从句动作先于主句动作.
　　When he had done his homework,he watched television.=He watched television when he had done his homework.
　　When he had painted the windows,he painted the doors.=He painted the doors when he had painted the windows.
　　但是,“When he painted the windows,he painted the doors.”这个句子则不能写成“He painted the doors when he painted the windows.”