네가 먼저 헤어지자고 했어

The line, explained

Picture two people under one umbrella that is not quite big enough, and one of them finally snaps: 'You're the one who said let's break up first.' It is a small sentence with a lot of history in it. Korean has a neat tool for this moment. Instead of quoting the other person word for word, it turns their old suggestion, 'let's break up', into a reported phrase with -자고 하다. Once you know it, you will hear it in every argument and every flashback.

네가 is 'you' as the subject. The plain word is 너, and in front of the subject marker 가 it becomes 네, so 네가. In real speech most people say 니가, partly so it does not sound like 내가, 'I'. 먼저 means first or before someone else. Here it is not about time so much as responsibility: it points the finger. 네가 먼저 is 'you, before me', which in an argument means 'this was your idea, not mine'.

헤어지다 means to break up or to part. Its 'let's' form is 헤어지자, made by adding -자 to the stem 헤어지. If someone says 우리 헤어지자, that is 'let's break up'. To report that suggestion later, you add -고, a small marker that wraps up quoted words, and follow it with 하다, to say. So 헤어지자 becomes 헤어지자고, and 헤어지자고 했어 means 'said let's break up'. The original suggestion stays inside the phrase almost unchanged.

했어 is the casual past of 하다. On its own 하다 means 'to do', but after a quoted phrase ending in -고 it means 'to say'. This is how Korean reports speech in general: the content first, then -고, then a verb of saying. Commands, questions and statements each have their own quote ending, and -자고 is the one for suggestions. Swap in other verbs and it works the same way: 가자고 했어, said let's go; 먹자고 했어, said let's eat; 기다리자고 했어, said let's wait.

Say it slowly with the stress on the accusation: ne-ga meon-jeo, then a breath, he-eo-ji-ja-go hae-sseo. The 자고 should sound clipped, as if you are holding up the other person's own words. Then try a gentler version from a happier scene: 네가 먼저 만나자고 했어, you were the one who asked to meet. Same grammar, completely different movie. That is the fun of -자고 하다: it lets you replay any 'let's' that anyone has ever said to you.