如果所谈论的事情完全发生在过去，则用过去时态(有时也叫做未完成时态)，例如：I used to live in Seattle /我过去住在西雅图。

If you are talking about something that is completely in the past, you use the Past Tense (sometimes called the Imperfect), e.g.

youdao

然而作为家长，我的生活重心完全发生了改变。

Being a parent, though, has certainly changed my priorities.

youdao

它也可能发生在完全不相关的、争夺相同资源的组织中，比如以种子为食的啮齿动物和蚂蚁。

It may also occur among totally unrelated forms that compete for the same resource, such as seed-eating rodents and ants.

youdao

And even though these things can come across-- can come apart slightly under certain scenarios, those details won't matter for what we're about to turn to.

即便这些东西-,在特定情况下不完全同时发生，这些细节不会干扰到我们将要探讨的问题。

耶鲁公开课 - 死亡课程节选

Now, you all know that we can make irreversible changes in proteins, you can denature them completely, that's what happens when you cook an egg for example.

现在你们都懂蛋白能够发生不可逆的改变,能够完全变性,比如在你煮鸡蛋的时候就是这样

耶鲁公开课 - 生物医学工程探索课程节选

Things started to change probably on the first day of class, but not entirely-- because there's shopping period and it's such a wonderful thing-- you shop for classes.

事情也许从开课第一天就发生变化，但没有完全不同-,因为这是购物期，妙不可言的阶段-,你们选择课程。

哈佛公开课 - 幸福课课程节选