A1
了 — the action finished
What 了 after a verb actually marks in Chinese, why 我每天吃了早飯 is wrong, and why a sentence about tomorrow can contain one.
了 after a verb says the action is complete. That is usually the past, which is why it gets taught as a past tense, and why so many sentences built on that assumption come out wrong.
我吃了。
Wǒ chī le.
I’ve eaten.
他昨天來了。
Tā zuótiān láile.
He came yesterday.
我買了三本書。
Wǒ mǎile sān běn shū.
I bought three books.
Note that the time word is doing the work of saying when. 昨天 is what makes the second sentence yesterday; 了 only says the coming was completed.
Why it is not a past tense
Two tests, and both fail if you treat it as one.
A habit in the past takes no 了, because a habit is not a completed event:
我每天吃了早飯。
我每天吃早飯。
每天 makes this a routine, and a routine never finishes. This holds whether the routine is present or past — 我小時候每天吃早飯 has no 了 either.
And an action in the future takes one as soon as one thing has to finish before another starts:
我吃了飯就去。
Wǒ chīle fàn jiù qù.
I’ll go once I’ve eaten.
Nothing there has happened yet. 了 is marking the eating as complete relative to the going, which is exactly what it does in the past too. Aspect, not tense.
The negative drops it
沒 already says the action did not complete, so 了 has nothing left to do and cannot stay.
我沒吃了。
我沒吃。
This is the most common 了 mistake, and it comes from English keeping its past marking in the negative — didn’t eat still looks past-tense. See 沒.
A bare object often wants a number
我買了書 is grammatical but feels unfinished, the way I bought book does — it sounds like the first half of something. Give the object a quantity, or add a second clause, and it settles.
我買了一本書。
Wǒ mǎile yì běn shū.
I bought a book.
我買了書就回家了。
Wǒ mǎile shū jiù huí jiā le.
I bought the books and went home.
The other 了
There is a second 了 that sits at the end of a sentence rather than after the verb, and it means something different — that a situation has changed. 我餓了 is not I hungered, it is I’ve got hungry. The two overlap in writing and are worth keeping apart in your head — see 了 at the end.