The general concept, for beginners, is that you trade pawns when you're lagging behind in development, and you trade pieces when you're ahead of your opponent in development. The concept behind trading pawns is that you allow your pieces more freedom to develop to their best squares (mobility), and that trading pieces, both major and minor, hinders your opponent's piece synergy/dynamics and possibly ruins their pawn structure. That is why, often times, Grandmasters choose not to accept minor piece trades (as in, they don't recapture and treat the intended exchange as a sacrifice). I saw a King's Indian game with Carlsen vs. Polgar, and Carlsen purposely declined a piece exchange.
PS: I don't recommend that you use the "Analysis" feature if you're a beginner. Just play without it and get a "feel in the blind," so to speak, about your playing style. Remember, the "Analysis" is meant to give you a computerised version of what you should have done, so don't take it too seriously.
It's a good question.
Unfortunately the question is nearly as complicated as "how do I play chess well?"
To get a good answer you'd need to post a game you played, and ask about a position where you were unsure what to do.