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  1. I heard about it. I have heard about it.

  2. I knew him. I have known him.

  3. He has played really well. He played really well.

marked as duplicate by Jon Hanna, James McLeod, anongoodnurse, RegDwigнt Feb 15 '14 at 14:11

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  • Really, this comes down to the basics of the past, perfect and pluperfect tenses in English, which would be a general reference (any book or resource on English grammar will cover this). I'd recommend this answer as being a nice summary. – Jon Hanna Feb 15 '14 at 12:54
  • Please search the site before asking. Here are a couple links for your convenience. This one provides a general overview of how the different tenses in English correspond to one another. This one, and the questions linked from there, addresses the choice between Present Perfect and Simple Past more directly. This one addresses an important difference between BrE and AmE. For more information still, we have a dedicated tag, which I have added to your question. – RegDwigнt Feb 15 '14 at 14:14
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The examples given are instances of present perfect tense contrasted with past tense. The present perfect generally emphasizes the present importance of past events, as opposed to a simple past tense, which in English may denote a completed action. Without more context it's hard to impute it here, but for the sake of argument, the simple past tense ("I knew him") might, by deemphasizing a present relevance, mean that the acquaintanceship ended or diminished at some point.

The third example ("He has played really well") seems to summarize the past (leading to a present state of affairs), while a simple past tense ("He played really well") might point to a specific (completed) past event.

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