. 主句谓语动词为现在时态，则从句动词可根据需要使用任何时态； 主句谓语动词为过去时态，从句的时态一般是过去范畴的时态，其中有如下几种情况要注意： 1. 当从句的动作与主句的动作同时发生，从句中一般用一般过去时或过去进行时。 2. 从句的动作发生在主句动作之后，从句时态用过去将来时。 3. 从句的动作发生在主句动作之前，从句时态用过去完成时。 ◆ 主将从现原则 ◆ 在if，unless 等词引导的条件状语从句； 在when，till/until，as soon as，before，after，as，while，each time，the moment等词引导的时间状语从句； 在no matter + wh-，however，whatever，whenever，even if/even though，so long as，on condition that等引导的让步状语从句中， 若主句为一般将来时（或包含情态动词/祈使句），从句谓语动词常用一般现在时表示将来； 若主句为过去将来时，从句谓语动词通常用一般过去时态代替过去将来时态。 e.g. I will call you as soon as she arrives. 当她来的时候，我就打电话给你。 If it clears up, we will go to the park. 如果天晴的话，我们就去公园。 I will not let them in, whoever they are. 不管他们是谁，我都不会让他们进来。 (2008全国Ⅱ，14) If their marketing plans succeed, they ____ their sales by 20 percent. A. will increase B. have been increasing C. have increased D. would be increasing ;.. . ◆ 特定引导词及句式的时态 ◆ Ⅰ. since since 引导的时间状语从句为一般过去时，主句中的时态用现在完成时或现在完成进行时。 e.g. Jenny has had another baby since I saw her last. 从上次我见到Jenny以来，她又生了一个孩子。 I have been learning English since I was in Grade 3. 自从上三年级以来，我一直在学英语。 The girl told her father she had been ill since she came home. 那个女孩告诉她父亲，自从回到家她就一直在生病。 Note: It + be + 一段时间+ since…中，若主句为一般现在时或现在完成时，从句常用一般过去时；如果主句为一般过去时，从句则常用过去完成时。 e.g. It is more than three years now since I saw her last. 自此我上次见到她，我们已有三年多没见面了。 It has been three years since I was in Beijing. 我不在北京已经三年了。 (1988全国) They asked me to have a drink with them. I said that it was at least ten years since I ____ a good drink. A. had enjoyed B. was enjoying C. enjoyed D. had been enjoying Ⅱ. before before引导的时间状语从句若是过去时，主句可用一般过去时或过去完成时。 e.g. He closed the door, before he left the classroom. 他在离开教室之前关上了门。 We had been to four cities before he came to join us. 他来加入我们之前我们已经去了四个城市了。 Note: before的常用句型 It will (won't) be + 时间段/ long + before + 一般现在时 ;.. 本文来源：https://www.wddqw.com/doc/64fdf87425284b73f242336c1eb91a37f111328c.html