了 / 着 / 过 / 在【le / zhe / guo / zài】

了, 着 and 过 go after the verb and 在 goes before it. None of them is a tense; each says how the action is seen. 了 marks it done, 过 marks it as something experienced at least once, 在 has it in progress, and 着 has the state it leaves behind continue.

了 or 过

了 reports that something happened. 过 says it has been part of your experience, and that the result no longer holds.

他去了北京 may mean he is there now. 他去过北京 means he went and came back. In a question, 你看过这本书吗 asks whether you've ever read it; 你看了这本书吗 asks about a specific reading you were meant to do.

在 or 着

在 shows an action going on. 着 shows the state that results from it, or the manner of a second action.

他在开门 is someone opening the door; 门开着 is a door standing open.

Combinations

过 and 了 can stack, in that order. 过了 says something is already over and done with – the meal, the exam, the date.

In speech, 在 and 着 can share a verb, with 呢 at the end, to stress that something is still going on: 他在打着电话呢. 了 and 着 never share one verb.

Negation

没 drops 了 but keeps 过 and 着.

• × 我没吃了 → 我没吃
• × 我从来没去上海 → 我从来没去过上海
• × 我昨天看着电影 → 我昨天看了电影