过 — ever having done it, against 了

規則 过 after a verb = have ever: 我去过中国。 It is about experience, not about when.
了 after a verb = one specific completed event: 我昨天买了一本书。
The negative is 没(有)…过, and the 过 STAYS: 我没去过中国。
Compare 了, which has to go: 我没买书。