I found that reading a lot helped a lot. For forum discussions, that would be reading other forum discussions, and observing the interactions between people. Languages have many fixed phrases to express intentions usally not found in dictionaries, which can be learned pretty well from observation. Don't concentrate on the content, but find out: when do people feel attacked, how do they defend themselves, how do they apologize, how do they phrase things carefully to refrain from judgement, or on the other hand how do they make their opinions clear? Things like that.
Also, when I write non-trivial posts, I look up almost all words I use, even those that I think I really know, to make sure I'm writing what I think I'm writing. My native language is german.
I don't think anybody ever died thinking they loved people too much, or had too much joy, or made too much music.
(If the post above is written in green, I'm posting as a moderator. If not, then I'm not.)