I found a sentence: The levels of biodiversity have decreased in the last three decades.
I thought the present perfect tense usually requires "for " or "since ", but this sentence has "in the last three decades" instead of "for the last three decades".
Could you teach me which I should use "for" or "in", or what is the difference?
Answer
If a figure has been decreasing for a period of time, it has been going down steadily over that time, like the water level in a leaky bucket.
Your sentence tells us that, over the past thirty years, every time scientists have calculated the level of biodiversity the figure has been lower. (It may have been decreasing steadily, but there was no way for them to measure that.)
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