了 — the particle that is not past tense

• Verb + 了 + (quantified object)
  The action was completed: 我买了三本书.
• … + 了 (sentence-final)
  Something has changed: 天冷了, 我十八岁了.
• 没 + Verb (no 了)
  Negation deletes 了 entirely.

Forget "past tense" — that instinct causes more 了 errors than anything else. 了 marks completion or change, and either can happen in the future: 我下班了就给你打电话。 (I'll call you once I've finished work.)

Verb-了 sits directly after the verb and says that action finished. It is most natural when the object is quantified or specific: 我买了三本书。 A bare 我买了书 feels unfinished — the listener waits for what happened next.

Sentence-了 sits at the very end and reports that the situation is now different from before: 天冷了。 (It's turned cold.) 我十八岁了。 (I've turned eighteen.) 我不吃了。 (I'm not eating any more.) Both can appear at once: 我已经吃了三个了。 — I've eaten three already (and that's the state now).

Two hard rules. (1) Negation deletes it: 我吃了 → 我没吃, never 没吃了. (2) Habits and states never take it — 我以前每天跑步 (I used to run every day) has no 了, because a habit is not a completed event.

For a past experience — something you have done at some point in your life — you want 过 instead: 我去过北京。 (I've been to Beijing.)

Want to hear these words in your own sentences? Paste any Chinese text into the Pinyin Annotator or the Chinese → Pinyin translator.

Is 了 the Chinese past tense?

No. Chinese has no tenses. 了 marks that an action is completed or that a situation has changed, and either can be in the future: 我吃完了饭就去 (I'll go once I've finished eating). Past time itself is carried by time words like 昨天.

What is the difference between verb-了 and sentence-了?

Verb-了 goes directly after the verb and marks that action as completed: 我买了三本书 (I bought three books). Sentence-了 goes at the very end and reports a change: 天冷了 (it's got cold), 我不去了 (I'm not going any more).

Why can't I use 了 with 没?

Because they contradict each other. 没 says the event never happened, so there is nothing completed for 了 to mark. 我吃了 becomes 我没吃 — the 了 simply drops out.

What is the difference between 了 and 过?

了 says an action was completed, usually a specific one: 我去了北京 (I went to Beijing — on that trip). 过 says you have had the experience at some point: 我去过北京 (I've been to Beijing). 过 is the one you want for "have you ever…?"