By the end of the 19th century it was becoming apparent that electricity had an intimate relationship with matter. With **by + a point in time**, the tense depends on whether you're talking about the **past**, **present**, or **future**. ### 1. By + past time → usually Past Perfect > **By 1900, scientists had discovered many new elements.** > > 到 1900 年时，科学家已经发现了许多新元素。 The action was completed **before** that past time. --- ### 2. By + present time → usually Present Perfect > **By now, you have probably understood the rule.** > > 到现在，你大概已经理解这条规则了。 The action is completed before the present. --- ### 3. By + future time → usually Future Perfect > **By next year, I will have finished my degree.** > > 到明年，我将已经完成学位。 The action will be completed before a future time. --- ### But "by" does not always require a perfect tense Your example: > **By the end of the 19th century it was becoming apparent that electricity had an intimate relationship with matter.** Here: * **By the end of the 19th century** sets the historical time frame. * **was becoming apparent** describes an ongoing process at that time. * **had an intimate relationship** is not an event that happened before then; it's a state that existed. So the writer uses **past progressive** (*was becoming apparent*) rather than **past perfect**. Compare: **By 1900, it was becoming apparent that electricity had an intimate relationship with matter.** **By 1900, scientists had discovered the electron.** The first describes a realization that was unfolding; the second describes a completed discovery. A useful rule is: > **By + time** often goes with a perfect tense when you're emphasizing that something had already been completed by that time, but other tenses are possible if you're describing a state, habit, or ongoing process at that time. -- "A little science distances you from God, but a lot of science brings you nearer to Him" - Louis Pasteur https://www.pinterest.com/pin/550494754462794176/ --

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