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Someone has been watching TV for three hours and he is still watching it.

I understand that I should say,

He has been watching TV for three hours

But is it correct to describe this situation by saying,

He has watched TV for three hours

Thanks in advance.
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But is it correct to describe this situation by saying,
<>He has watched TV for three hours
It is acceptable, but not as precise as the alternative "has been watching".

"has watched for three hours" can mean "on another occasion" or "on other occasions", because "has watched TV" can mean "has had a TV-watching experience" (which "has been watching" cannot):

He has watched TV for three hours three times this week -- on Monday, on Wednesday, and on Thursday.

"has watched" treats the situation as an event; "has been watching" treats it as an activity.

By using "has been watching" you make it clear that the activity of watching continued up to the time you uttered the statement.

CJ

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Nice explanation, CaliJim, as usual.
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Hi!

(A Practical English Grammar - Thomson, Marinet): An action which began in the past and is still continuing or has only just finished can, with certain verbs, be expressed by either the present perfect simple or the present perfect continuous. Verbs which can be used in this way include: expect, hope, learn, lie, live, look, rain, sleep, sit, snow, stand, stay, study, teach, wait, want, work.

But I think that when we use watch as a voluntary action (not as a verb of the sense) we can use both tenses for a still continuing action but I'm not sure because I'm not a native speaker.

To use the Present Perfect Continuous is better...Emotion: smile
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Thank you dear Peťulinka and thank you dear CalifJim for this wonderful explanation.