Learn Turkish: Çıkarcı arkadaşım gitti o gidiş, geri döneceği yok. - My selfish friend left and that was that, he has no intention of returning.

This is the core idiom of this lesson. It uses the past tense of a verb (gitti - he left) followed by o (that) and the verbal noun form of the same verb (gidiş - the act of leaving). Together, gitti o gidiş implies an irreversible action: he left, and that act of leaving is final. You can do this with other verbs too, like uyudu o uyuyuş (he fell asleep and he's been out cold ever since).

If I wanted to change the subject to 'I', how would the gitti o gidiş part change?

The second half of the phrase actually stays frozen. You only conjugate the first verb to match your new subject. So, if you were talking about yourself, you would say gittim o gidiş (I left and that was that). The phrase o gidiş remains exactly the same.

Does the word çıkarcı carry a more specific meaning than just 'selfish'?

Yes. While 'selfish' is a natural translation here, çıkarcı specifically means 'opportunistic' or 'self-serving.' It is built from çıkar (benefit, personal interest) plus the habitual suffix -cı. A çıkarcı friend is someone who only sticks around when they can extract a benefit from the relationship.

Why use döneceği yok instead of simply saying geri dönmeyecek (he will not return)?