If I'm downloading something and it should take 2 hours to be downloaded. So after one hour for example i would say, One hour has passed Or One hour passed And why ?
You're much more likely to say One hour has passed. This context is a good example of when we use present perfect. The action clearly has a relevance to the present time as the downloading is still in progress at the time of speaking.
You'd (normally) use the past simple if you were narrating the event: He sat in frustration. An hour passed, but his files were still downloading. From the participant's perspective, the present perfect is the natural choice.
If we're going to talk about natural English, a native speaker would be unlikely to use "an hour has passed" in that situation at all. I'd say "It's been downloading for an hour and there's still an hour to go" or "I've been downloading it for an hour and it still has an hour to go!"
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