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What's the difference in meaning between the present perfect and present perfect continuous tenses?

For example:

(1) She has worked here for 20 years (present perfect)

(2) She has been working here for 20 years (present perfect continuous)

The only difference I can find is that in sentence (1), she may not be working here henceforth, whereas in sentence (2), the implication is that she still works here. Is this the only difference? I don't think sentence (1) implies she definitely no longer works here, and if she does still work here, then the meanings of sentences (1) and (2) are the same.
Comments  
The difference - or the amount of difference - is connected with the particular verb in a particular usage. Some verbs show little distinction (like "work"). The difference is more obvious with other verbs.

I've watched Casa Blanca. (at some time before now - maybe even two or three times - I had the experience of watching that movie from beginning to end - I had a "watching-that-movie" experience.)
I've been watching Casa Blanca. (and am still watching it - I am currently undergoing the "watching-that-movie" experience - I've just now paused in that activity to tell you about it.)

The non-continuous form is usually consistent with an event or accomplishment interpretation.
The continuous form is consistent with an activity interpretation.
Well said, Calif Jim. I've encountered lots of these unusual nuances and have tried to understand it, but I like your succinct explaination.
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1. Amy has been writing the letter for two hours.
2. Amy has written the letter for two hours.
#1 is supposed to be correct. Is #2 also acceptable?
# 1 sounds fine to my ear.
# 2 - is sounds a bit stiff due to the recent time, "2 hours". Pres. perf works better with a longer time span.
I have studied English for 15 years.

It's fine If it's: Amy has written letters to her boyfirend in English for two months but without response.
sitifan1. Amy has been writing the letter for two hours.
2. Amy has written the letter for two hours.
#1 is supposed to be correct. Is #2 also acceptable?

I don't find that #2 can be contextualized in a way that makes it idiomatic and acceptable, no.

These would strike me as correct:

Amy has been writing that letter for two hours.
[continuous application of effort at that task until the moment of utterance]
Amy has written letters for two hours [today / every day this week]. [one block of time occupied by the activity of letter writing; once or repeated, depending on the adverbs]

CJ
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3. Amy has written three letters for two hours.

4. Amy has written three letters since lunchtime.

Are the above sentences acceptable?
3. No.
4. Yes.

CJ