both of them are used for past tens, but there is a little difference. the 了 emphasize the things finished, the things is done. the 过 emphasize the things past through, and it was a experience, and the time when you finished it was not important. For example, 你吃过饭了吗？----------吃了（when you ask anyone "did you have lunch?", everyone knows you asked the recent lunch, not last Monday lunch, so they use 了）. 你去过美国吗？——去过（that means you visited USA before, you have the experience, but the time when you visited was not concerned ）

过，remote past。了，recent past。了，不能用于否定句

disclosure: I am an advanced but non-native speaker Neither 了 nor 过 express "tense" in the way that Indo-European languages like English use tense. Tense is strictly about verbs and their relationship to time; but both 了 and 过 are not bound by considerations of time. 过 is an "experiential marker"; it denotes that the verb has been or will have been completed, and that its completion has bearing on its overall significance to the speaker. Similar in many cases to "have done" or "have [verb]-ed". But, like I said, it's not always about tense. For instance: • 我学过中文 'I have studied Chinese' -- 过 here is an experiential marker, but if we look closely in English, we'll see that sentence is still present tense (past tense would be "I had studied Chinese"). • 买甜点呢，得先问过爸爸！‘If you want to buy sweets, you first need to ask dad!" Notice here that this is a present/conditional use of 过. 了 has several unique meanings in linguistically distinct categories. One is "the change of state"--a marker, regardless of tense, that some relevant situation has or is changing. For example: •  已经十点了！‘It's already 10!' • 你的咖啡来了 'Your coffee is here/Your coffee has arrived" -- note that the translations into English can be variable, and don't necessarily require a certain tense. 了 is also what linguists call a perfective marker, something that marks the completion of a verb. However, completion does NOT just occur in the past tense. • 昨天我看了两本书 'Yesterday, I read two books' -- Here, the event is 'past-tense', but we only know that because of 昨天, not necessarily because of 了 • 到了学校以后，学生就得去教室。'Right after arriving at school, students must then go to the classroom.' Here, 了 works in conjunction with  以后 and 就 to express that immediately after 到 'arriving' is completed, the subsequent action (going to class) must occur. But again, this is not a past tense phrase. Those two particles are very difficult, and it's tempting to view them in terms of tense. But linguistically, that isn't accurate!

although I am a Chinese, but I don't know if my answer is correct or not. 1. V.+ 了+ sth.... = Did something. 2. V.+ 过+ sth.... = Have done something. 3. "了" can be a particle: V.+ 了. = Something just V.ed.