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“主过从必过”是什么？
之前推送过文章里的一句话（语法知识分享｜看似复杂的非谓语也有规律）： I hear they've promoted Tom, but he didn't mention that he had been promoted when we talked on the phone. 改写为从句之后是：
I hear they've promoted Tom, but he didn't mention that he had been promoted when we talked on the phone.
这句话里面有好几个不同的时态：
I hear 一般现在时
they've promoted 现在完成时
he didn't mention 一般过去时
he had been promoted 过去完成时的被动语态
we talked 一般过去时这么多时态是要吓死人！怪不得学生语法学起来那么困难。
咱们先看句子结构：
but 前面是宾语从句。
but 后面是另一个宾语从句 + 时间状语从句。
这俩宾语从句正好可以讲解宾语从句部分最常考的时态问题。
有个好用的时态口诀：主现从随便，主过从必过，客观真理永一现。（感谢前人总结的口诀）。
什么意思呢？▼
说的是在宾语从句中：
如果主句是一般现在时，从句的时态不受限制，根据具体情况什么时态都可以；比如前半句：I hear they've promoted Tom（主句一般现在时，从句是现在完成时）；
如果主句是过去时，那么从句一定得是过去的某种时态（一般过去时、过去完成时、过去将来时）；比如后半句：he didn't mention that he had been promoted（主句是一般过去时，从句是过去完成时）。要是用 he has been promoted 就错了。
如果表达的是客观真理，则管你主句是什么时态，从句永远是一般现在时；比如：The teacher told us that the earth goes round the sun.
这个口诀特别好用，大娃一定要记住并会用，考试上用得着。
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