Chinese marks aspect - how an event unfolds - rather than tense. Three particles do most of it, and they attach to the verb or end the sentence.
了 - completion or new situation
Two jobs hide in one character. Verbal 了 marks a bounded, completed event: 我吃了三碗饭 (I ate three bowls - done). Sentence-final 了 marks a new situation or change: 下雨了 (it's raining - it wasn't before), 我饿了 (I'm hungry now). The famous trap: 了 is not past tense - 我明天就回家了 talks about the future just fine, because the event is bounded, not "past".
过 - experience
过 marks "have had the experience of": 我去过中国 (I have been to China - at least once), 你看过这个电影吗？ Negate with 没（有）: 我没去过. It says nothing about when - it is a life-resume marker.
着 - ongoing state
着 marks a state in progress, usually static or durative: 门开着 (the door stands open), 他戴着眼镜 (he is wearing glasses - the state), 桌上放 着一本书 (a book lies on the table). Continuous "doing" more often uses 正在/在： 他正在吃饭.
Quick reference
| Marker | Marks | Example |
|---|---|---|
| 了 (verb) | completed event | 我喝了一杯咖啡 |
| 了 (final) | new situation | 我饿了 / 他走了 |
| 过 | experience | 我去过三次 |
| 着 | durative state | 灯亮着 |
The practical study tip: do not drill the particles as isolated rules. Learn them attached to the five or six verbs you use daily - 吃了, 去过, 住着 - and let the pattern generalize. Aspect marking is one of the few grammar areas where a dozen real sentences beat a chapter of theory, and the pill decks carry them in that shape.
Common questions
Is 了 a past tense marker?
No - it marks completed/bounded events or a change of situation. 我明天就走了 uses 了 about the future. Time words carry tense; 了 carries aspect.
What is the difference between 了 and 过?
了 marks a concrete completed event (我吃了饭 - I ate); 过 marks accumulated experience (我吃过 - I have eaten it before, sometime in life).
Where does 了 go in a sentence?
Two positions do two jobs: right after the verb (我买了那本书) or sentence-finally (他结婚了 / 我不吃了). Many sentences carry both flavors of meaning; context resolves it.