Ok, so the first two common uses of 了 (final sentence 了， change of state) and completion 了 are fairly straightforward, and even when they're not, they usually 不会成问题。
But then, there is the 持续 了， which in reality is not a third use, but rather an extension of the final sentence/change of state 了。 But, for now we'll say it is a third 'use' at least.
From Rimmington and Ching's Intermediate Chinese, I found this interesting comment on their take on this third (of course there are many uses of 了， but you know what I mean) use of 了。
"Sentence 了 le naturally often occurs in sentences which include a verb-object or verb-complement phrase marked by the aspect 了 le. In these cases the speaker, by using sentence 了 le, is adding his or his gloss to the statement or question. Consider the following pairs of examples in which the first is a statement of fact and the second adds the speaker's comment: (italics added by me)
我们等了两个钟头。 - We waited two hours.
我们等了两个钟头了。 - We have been waiting/have waited for two hours.
她去了三次。 - She went three times.
她去了三次了。 - She has been there three times.
我们看了那个电影。 - We saw that film.
我们看了那个电影了。 - We've seen that film.
他喝了十啤酒。 - He drank then glasses of beer.
他喝了十杯啤酒了。 - He's had ten glasses of beer (and that's why he can't stand up).
This is their explanation of the language point you likely learned as:
我学了两年汉语。 I studied chinese for two years.
我学了两年汉语了。 I've been studying chinese for two years.
Now, something has struck me as off about this whole thing for a long time, and I'm glad Rimmington and Ching at least try to illuminate it, however badly, with this notion of a 'gloss' over the previous utterance.
Real English in use:
How long have you been studying chinese for?
Two years. I've been studying for two years. I studied for two years, (so far). So far, I've studied for two years.
All four of these responses are totally acceptable and while you can read some of them with a heavier emphasis on past completion or continual study, depending on context, inflection, etc, this distinction, while existing in english, is hardly critical in a wide variety of contexts. Chinese places an emphasis on this. Bothersome. Why? Because completion 了 is very strong.
Back to the more.
Their translation of "have waited." Seems iffy at best.
If one waned to say "We've waited for two hours." that seems to me to be an 已经 sentence even if the english omits the 'already'.
While I see the 持续 element in all the sentences, it's the third example:
that strikes me as the most troubling. Troubling, because if this really is
We've seen that movie.
Then this subtle use of the 持续 final 了, needs to be given a very serious treatment in language textbooks because it strikes heads-on with two related sentences.
Ok, if we are to take the 过 to be an emphasis marker, then we must accept that the english sentence
We've seen that movie.
can be read two different ways.
1, with an emphasis on the continuation into the present and it's effect in the present on the situation regarding the speaker's utterance.
2, with an emphasis on the completion of the action.
But, because of how english grammar works, this is going off the hill. Why?
Because english verb use is either nonsensical, or it simply allows 持续 in a wide variety of contexts with different verbal conjugations.
There is no reason to assume that in response to
Do you want to see that movie?
We've seen that movie (before).
We saw that movie (before).
carry any different level of 持续 just because one utilizes the word "have" or it's contraction "'ve" denoting a perfect (in this case, present perfect) tense. The influence of that completion is equally relevant and "coninuing" onto the current situation of the speaker/situation.
The same idea in some sense includes the length of study example.
How long have you studied for?
I've studied for two years. (I studied for two years.)
In the second case, an addition of "so far" at either end makes this very clear that the verb use is not the determining factor in reading english grammar, while the 了, being an aspect marker, but the closest thing chinese gets to a verb conjugation, is decisive in the reading of the sentence.
Back to the top
认识 as a verb only deals in fixed points and hence can't be 持续'd, hence the 了 aspect marker here is not our 持续 了， but rather a 变化 了, or perhaps a special use of 了 that accompanies the use of 久。
熟了 is clearly both a completion and a change of state, but not a 持续。
What about number 3?
Number 4, What's weird is this 了 is emphasizing the action, 相处 a 持续'able action (in chinese, the condition and verb need to be something is capable of continuing and flowing, with 熟悉 the verb can't be 'flowed' and only brings about a state, a new condition, the same problem with 认识) continuing into the second clause's condition as a precondition for it.
Again, already started to 相处 with someone (we'll call it 'get to know' or 'get along with'), continue doing that and it will bring about whatever clause two says.
Why 3 makes things weird. It forces you to resist thinking the 了 signals simply a 变化.
Translation of 3/4 ： Once/After you've spent a lot of time with him, I'm sure you'll like him.
ISSUE: this 持续 'have' 'pefect' aspect in english is so weak here as to be almost non-existent. Why? I don't know! I think in English the main idea here is the completion, the arrival at a new point, the change in the status, which is not what the 了 in the chinese signifies. Yes, the 'have' in english denotes perfect aspect, and the 了 in chinese appears to be a perfect aspect marker in this case, but that idea is not important in english.
After you finish loading those boxes you can go play with your friends.
After you've finished loading those boxes you can go play with your friends.
Yes, the 2nd holds the perfect aspect, but does it matter? Sure, if you are hip to grammar and think about these things you can see what the perfect aspect means, why it exists, originated, etc, but in real use, the important thing this sentence and situation is the completion, not the aspect. I would imagine this is why english doesn't stress this difference in certain situations and why I felt impelled to write a 100,000 word post on it.