了, 过 or 着

Chinese uses three small particles right after the verb to show how an action happens.

• 了: the action is completed, often before the next thing happens: 下了课… (after class…), 到了北京就… (as soon as you get to Beijing…). It also goes with amounts and 已经: 等了半个小时 (waited half an hour).
• 过: you have had this experience at least once. Typical cues are 从来没…过, 以前…过吗 and 曾经…过: 我从来没去过日本 (I have never been to Japan).
• 着: an ongoing state, or the way another action is done: 躺着看书 (read lying down), 笑着说 (say with a smile), 门开着呢 (the door is open).

Tip: if a second verb follows and the first verb tells you how it is done, use 着. If the first action has to finish before the second one starts, use 了.