Question

One study found that students' grades________(improve) a little after ...为什么不能用过去将来时？

One study in America found that students' grades________(improve) a little after the school introduced uniforms.
为什么这里不能用主将从现，填would improve啊？

Answer 1

回答如下：
本句中的主句动作发生在过去，宾语从句的动作发生在时间状语从句动作之后，故该空应该填一般过去时improved; 而过去将来时则表示该动作当时还未发生。类似例句：
He went home after he (had) finished his homework.
He said he would go swimming after he finished his work.

Answer 2

One study in America found that students' grades would improve a little after the school introduced uniforms.
that引导的宾语从句是过去将来时，因为主句时态是过去时态，这里有时态的呼应。
主将从现用于时间、条件状语从句。

Answer 3

因为这个句子是宾语从句，主句that前面是found，用的是过去式，所以后面的从句也要用过去的某种时态，本题根据意思只能填写would improve.

Answer 4

One study in America found that students' grades would improve a little after the school introduced uniforms.
这句话中that之后是宾语从句。因为前面的主句是过去时，宾语从句。应该使用过去的某种时态。而这里的改善是在found之前，所以应该用过去完成时

Answer 5

一般虚拟式表现的是一种推测，而上面这句话的整体意思来看，是已经得出了结论，所以用improved，也符合主句中动词found的时态。