所谓的“主将从现 ”的第一层次，即 “主句用将来时（现在将来时和过去将来时），从句用一般时（一般现在时和一般过去时）”,这一层次也可以称作“初中层次 ”第二层次：“主将” 中将来时有将来进行时，将来完成时，将来完成进行时，而“从现” 中相应的时态也可能是现在进行时，现在完成时，现在完成进行时例如：If the snow has completely stopped, we will be playing with snow on the playground.如果雪能够完全停下来，我们将正在操场上玩雪呢！If it is snowing, we will have been walking in the snow for more than two hours.如果天一直在下雪，我们将在雪地里走了两个多小时了另外，引导条件状语从句的连接词（connector）还可以是 unless, so long as, in case, on condition that, supposing that, provided that 等if 引导条件状语从句的两种用法：第一种用法：当某事发生的可能性较大时，这种情况，有个十三字口诀：“ 主将从现，主过将从过，主现从现 ”。
即主句是一般将来时，从句用一般现在时；主句用过去将来时，从句用一般过去时；当主句中有 can/can’t 的话，则从句用一般现在时例如：If he has time, he will come tomorrow.If she finishes early, she can come back early.If you don’t do homework, the teacher will scold you.第二种用法：1.if 从句还表示不可实现的条件或根本不可能存在的条件,也就是一种虚拟的条件或假设,从句多用一般过去时或过去完成时如:If I were you, I would invite him to the party.如果我是你,我会邀请他参加聚会I would have arrived much earlier if I had not been caught in the traffic.如果没有堵车,我会到的早一点儿2.but for, without 等引出一个介词结构来表示条件 ,条件常常是虚拟的,或与事实相反的假设如: but for 若非,要不是But for the rain, we should have a pleasant journey.要不是下雨,我们的旅行肯定会很愉快。
But for your help,we should not have finished in time.要不是你帮忙,我们肯定不能及时完成注意：除了 if 之外,其他连词也可以引导条件状语从句A.Unless conj.除非,若不,除非在……的时候You will fail to arrive there in time unless you start earlier. 如果你不早点动身,你就不能及时赶到那儿Unless it rains, the game will be played.除非下雨 ,比赛将照常进行B.supposing conj.如果,假如supposing 引导的条件从句表示一种假设条件Supposing it rains, shall we continue the sports meeting?倘若下雨,我们的运动会还要继续举行吗?Supposing something should go wrong, what would you do then?假如出了什么问题,你准备怎么对付?。