compare:1. 你吃过饭了吗？ 2. 你吃过蛇肉吗？ so, “过” in sentence one is different from the "过“ in sentence 2, the first one means "completed", while the second one means "experience". 1. "过” indicate the meaning of "completed" , can be followed by the particle “了”. 晚饭已经吃过了。 "过” indicate the meaning of "experience", can't be followed by "了“. 蛇肉我吃过。 2. can't add the word “以前” before "过” that indicate the meaning of "completed"，but the second 过 is ok with it. 晚饭已经吃过了。 蛇肉我以前吃过。 3. the way of answering the question is different. A: 你吃过了吗？ B: 吃过了。/还没吃（呢）。 A：你吃过蛇肉吗？ B：吃过。/（还）没吃过。

放在动词后面，表示done

In some ways，过 and 了 have the same meaning. 我已经吃 过 晚饭 我已经吃 了 晚饭 Their meanings are both that i have already had supper. But... 我到过那儿（I have been there） 我到了那儿（I have just arrived there）

我去过北京（I have the experience to Beijing,but I'm not in Beijing right now） 我吃过臭豆腐（I have the experience to eat...） ---你昨天去哪儿了？----我去北京了（Maybe I am still in Beijing now.)