过

过 has two main jobs that beginners must keep separate. (1) As the experiential aspect particle (toneless guo), it goes right after a verb to mean 'have ever done X (at least once in the past).' 我去过北京 = 'I've been to Beijing (at some point).' Negate with 没 + verb + 过: 我没去过 ('I've never been'). This is different from 了, which marks a completed event tied to a specific occasion: 我去了北京 = 'I went to Beijing (and we both know which time).' (2) As the full verb 过 (guò, fourth tone), it means 'to pass / cross / go through' — 过马路 (cross the street), 过桥 (cross a bridge), 过生日 (celebrate / 'pass through' a birthday), 过年 (celebrate New Year). Same character, two readings, two grammars. In conversation, the particle 过 is what makes 'have you ever...?' questions possible — 你吃过饺子吗？('have you ever eaten dumplings?') is one of the most useful HSK 2 patterns.

Note: Neutral and extremely common. As the aspect particle, 过 is always toneless (guo); as a full verb 'pass / cross / celebrate,' it's fourth-tone guò.

Memory hook: Think of 过 as 'passing through' an experience — you walked across it once, even if you can't say exactly when.