We use the simple past tense to talk about what happened at a definite time in the past.

我们用一般过去时来谈论过去的某一事件发生的事情。

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Since the 1920s, scientists have known the universe is expanding, which means it must have started at a definite time in the past.

自20世纪20年代以来，科学家已经了解到宇宙正在澎张。这就是说宇宙一定是在过去的某一特定时候开始形成的。

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He chooses at a definite time and place under definite conditions between a strictly limited quantity of gold and a strictly limited quantity of iron.

他是在一定时间、一定地点，在确定数量的金和确定数量的铁之间作选择。

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The early promise of unlimited cash from the Murdocks looks less definite at a time when backers consider cancer vaccines, which attracted a lot of money over the past decade, "dead as an investment, " in the words of Ivor Royston, an immunologist and a partner at San Diego venture capital firm Forward Ventures.

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