before引导的从句为何不需要通常目前年代替以后时

It wont be long before his boy will help the girls with their bags.

It wont be long before his boy will help the girls with their bags.

【翻译】用不了多长时间，他儿子就会想帮那些女生拿行李袋了。

【答】在时间、条件、让步等状语从句中，即使两个动作都是以后发生，语法也不允许主从句都用以后时。规定从句用通常时才代替以后时。

在before his boy will help the girls with their bags 这个时间从句中，will help意思是“想帮助”。will是情态动词，表示人的意愿、愿望；不表示以后。又如：

If youll excuse me for a while, Ill find out for you. 请稍候一会儿，我给你查一下。（你若想等的话）

If you will keep waiting for a while, Ill help with your enquiry. 请你再等一会儿，我要帮你解答一下。