A: 아, 그렇구나. 그래야겠다. 조만간 다 같이 한번 보자. Oh, I see. I should. Let’s meet up all together one of these days. B: 그래. 한번 뭉치자. 본 지 진짜 오래 됐다. Sure. Let’s get together. It’s been a really long time since we last met. Particularly in the last sentence of what person B said they used the infinitive 다 form of the verb 되다. Does it make any difference in meaning when keeping it in this form? =first, in this situation, 본 지 진짜 오래 된것 같아. 본 지 진짜 오래 된것 아니야? 본 지 진짜 오래 됬네. all those sentences have the same meaning, emphasizing we haven't met for a long time. but precisely, 본 지 진짜 오래 된것 같아. (this has the speaker's view.) 본 지 진짜 오래 된것 아니야? (the speaker wants a listener's reply) 본 지 진짜 오래 됬네. (it is almost the same nuance 본지 진짜 오래 됬다.) last one and your example's sentences have a nuance that announces or making sure the fact that all of the conversation participations know. so the speaker doesn't expect A will reply to it. Does it show that you’re saying it to yourself? Umm... kind of. but It is more similar saying objective fact to the listener, like news announcer. and you probably heard about specific dialogue form which is for talking to oneself, but I don't know how to explain it clearly, so i will leave one example and youtube link which is about it 선생님: 재호 오늘 시험 잘 봤네! Teacher: Jae ho, you got a high score in this test! 재호: 문제가 쉬웠나? 감사합니다! Jaeho: maybe cuz of easy problems?(to my self) Thanks! youtube: https://www.youtube.com/watch?v=7gaVhmTlvkM How do I know when I can use this form in casual conversation? umm with context, sorry I will also search about it and leave a comment of it. And does saying a causal sentence like this in the infinitive form add an underlying nuance to the sentence? not very much I think, but we don't use it to elder people, sometimes use it but not much. and also when i got new information of it I will leave a comment too and if you have a question of this, plz leave a comment^^ thanks!