https://lichess.org/training/3eMiC
Lichess tactic trainer: Find the best move for white.. Played by 16236 players.
I'm a new chess player and do not understand why the solution to this puzzle is Qc2+ instead of something like Rb8, Qf5, g4 I don't really understand what black could do here to save the Queen?
Especially when looking at puzzles, always look for forcing moves (like checks) because it forces your opponent to react and makes it easier to calculate for you. In this case, checks can be done by the queen or knight.
This forks the Rook and King and forces Black to lose their rook after Qxa4. Rb8 like you suggest does not win any material, and is not forcing your opponent to make a specific move. Hope that helps. ---
Qc2+ forks the rook on a4 and the king, so black must either move the king to the back rank where white's rook can attack it or otherwise offer the Queen ...Qb6 (which is what the computer plays). Could also play g6 but either way, by playing Qc2+ and forking the rook in check you will play Qxa4 and take the enemy rook on the next move and black is pretty much done
Usually every time you feel like you don't get the solution, as it seems there's a better move, it's because you're missing some counter to the "better" move. It gets easier with time to spot those moves early on, but the harder the puzzles get the harder it becomes. It never ends. Welcome to chess!
Why would Rb8 force Qf5?
The solution given by lichess nets you a rook, I don't see a way to take the black queen
As Joe said, the problem is that these moves are not forced. After Rb8 black has a few options to play, like moving the rook out of the danger. And in case white follows with Qc2+, black can also defend with either Qg6 or g6 and keep the queen safe or exchange. The puzzle solution in this case is a double attack that wins material for white regardless black's response.