A2
了 at the end — something changed
The second 了 in Chinese — the one at the end of a sentence, which says a new situation now holds — and how to tell it apart from the 了 that marks a finished action.
There are two 了s. One sits right after the verb and says the action finished. This one sits at the end of the sentence and says something is now different from before.
我饿了。
Wǒ è le.
I’m hungry.
下雨了。
Xià yǔ le.
It’s started raining.
我二十岁了。
Wǒ èrshí suì le.
I’m twenty now.
None of those are about a completed action. 我饿了 does not report an event of becoming hungry; it says hungry is the situation now and was not before. The closest English gets is now or the perfect — I’ve got hungry, it’s started raining — which is why the plain translations above quietly lose something.
Adjectives take it without 很
An adjective normally needs 很 to stand as a statement. With this 了 it does not, and adding one pulls against the meaning.
我很饿了。
我饿了。
很 describes how things are; 了 says they have changed. Pick the one you mean — 我饿了 for I’ve got hungry, 我很饿 for I’m very hungry.
太…了 is the exception, and it is a fixed pattern rather than these two rules interacting: 太贵了, that’s too expensive.
With a negative it means “not any more”
This is where it is most useful, and where English needs three or four extra words.
他不来了。
Tā bù lái le.
He’s not coming any more.
我不喝了。
Wǒ bù hē le.
I won’t have any more.
Note that this 了 sits happily with 不, which the completed-action 了 never does. The plan changed; that is a change of state like any other. 我不去了 said at a party means I’ve decided not to go after all.
Both in one sentence
They can appear together, doing their separate jobs — the action finished, and that is news.
我吃了饭了。
Wǒ chīle fàn le.
I’ve eaten.
The first 了 completes the eating; the second says the situation has moved on, so you need not be fed. It is a mouthful and very common.
Which one is it?
If 了 comes immediately after the verb, it is the completed-action one. If it is the last thing in the sentence, it is this one. When the verb is the last thing — 他来了 — the sentence is genuinely both, and no one bothers to separate them.