3...c5 gets two question marks deservedly, not because it loses material, but because it is extremely antipositional and a strong player would win as White on autopilot. The c5 pawn is weak itself, it blocks the Bf8
, it can no longer protect the squares d5, b5 and d6. Any compensating "grip" on d4 is an illusion. It does surprise me a bit that the machine gives an advantage as great as +2.0 but I would really hate to have the Black position against a strong opponent. ..c5 is not a tactical blunder but a positional blunder.
An advantage of 2.0 does NOT mean that you can immediately win two pawns. White could easily give the advantage away, but with correct play Blacks position will gradually collapse under the weight of its own defects. I would start with 6.Nc3, eyeing the weak squares at d5 and b5. These are not so serious at the moment because if the Knight actually goes to either square Black can retreat Bd8, but later on it might be annoying. And if Black makes the natural reply 6..Nf6, White has 7.e5. So to prepare that move Black might try 6...d6, but then 7.Bf4 to be followed by 0-0-0.
Rather than attempt to analyse this position I suggest you try taking Black against your computer. Allow yourself as many takebacks as you like. I predict that you will be squashed. You will not be able to find anything constructive to do. Take note of how the machine exploits the weaknesses that I have mentioned.
Now give yourself the White pieces and see if you can give the computer a dose of its own medicine. Again give yourself any number of takebacks. I predict that the thrill of victory will be yours. I predict also, and certainly hope, that you gain a better understanding of what it means to hold a positional advantage.
