过 normally need follow to a verb, for example, 吃过,看过,去过etc, and it indicates that the action happened in the past and have completed; whereas 了is used in the end of the sentence, and make the response sound gentle. and also sometimes u can use them in the same way, such as 你吃过吗?= 你吃了吗。 the best way is chat with Chinese people, and then with the exercise going on, u will be familiar with their usage. keep practising!