널 사랑하기 때문에 보내는 거야

The line, explained

The train is about to leave, and one of them is not getting on. 널 사랑하기 때문에 보내는 거야, 'I'm letting you go because I love you'. It is the noble-sacrifice line Korean melodrama does better than anyone. The pattern giving the reason is -기 때문에, a firm and serious 'because'. Where -아/어서 links a cause casually, -기 때문에 sounds like an explanation the speaker wants you to hear and remember.

널 is 'you' as the object. It is a squeezed form of 너를, very common in speech and lyrics. 사랑하기 comes from 사랑하다, to love. Add -기 to the stem, the same -기 you met in -기 전에, and you get a noun-like 'loving'. Then 때문에 follows: 때문 means 'reason' or 'cause', and 에 marks it. So 사랑하기 때문에 is literally 'for the reason of loving'.

The rule is simple: any stem, plus -기 때문에. 바쁘다 → 바쁘기 때문에, because I'm busy; 아프다 → 아프기 때문에, because it hurts. For a past reason, put the past marker on first: 사랑했기 때문에, because I loved you. With a noun, drop the -기 and use the noun directly: 너 때문에, because of you; 비 때문에, because of the rain. That short form is everywhere in daily life.

보내는 거야 completes the thought. 보내다 means to send, but with a person as its object it often means 'to let go' or 'to see someone off'. -는 거야 is a casual way of saying 'it's that…', used to explain what is really going on. 보내 alone would be 'I'm letting you go'. 보내는 거야 says 'the truth is, I'm letting you go', as if answering a question nobody dared to ask.

Say it slowly, as the whistle blows: neol sa-rang-ha-gi ttae-mu-ne, then quieter, bo-nae-neun geo-ya. Then try other reasons: 널 믿기 때문에, because I trust you; 널 아끼기 때문에, because I care about you. And remember the everyday short form: 너 때문에 웃어, I smile because of you. Said with a different face, 너 때문에 can also mean 'this is your fault', so watch the tone.