这两个句子为什么用不同时态？
有这样两个句子：(1)The teacher urges his students to keep working at their pronunciation until they've improved it.
(2)The teacher tells his students to read the essay over and over again until they understand it.
为何一句用现在完成时have improved， 而另一句却用现在一般时understand?
主要跟动词的性质有关，understand是所谓的瞬间动词。
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