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。
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。
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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