过: have done before (experience)
动作的态 / aspect (经历)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Subject + Verb + 过 + (Object) / 没 + Verb + 过
过 (guo) after a verb expresses an experience you've had at least once: 我去过中国 = I've been to China (it's part of my life experience).
Difference with 了: 我去了中国 = I went to China (a specific event; I may still be there); 我去过中国 = I've been to China (and came back).
Negative: 没(有) + V + 过 (keep 过!): 我没吃过北京烤鸭. Question: 你…过吗？/ 你…过没有？
Examples
你去过长城吗？
Nǐ qùguo Chángchéng ma?Have you ever been to the Great Wall?
我吃过一次北京烤鸭。
Wǒ chīguo yí cì Běijīng kǎoyā.I've had Peking duck once.
我从来没坐过高铁。
Wǒ cónglái méi zuòguo gāotiě.I've never taken the high-speed train.
他以前学过两年中文。
Tā yǐqián xuéguo liǎng nián Zhōngwén.He studied Chinese for two years in the past.
Common mistakes
✗ 我没去过了 → ✓ 我没去过.
我没去 (= I didn't go, this time) ≠ ✓ 我没去过 (= I've never been).