Here in this video https://www.youtube.com/watch?v=F2MkdD1B77E around the 18:30 mark, I have played this in the last couple decades as black and thought that the position was lost for black, but actually the computer gives it as small advantage to white. Which if you don’t understand the value of positions means that black still has good chances to draw the game. My early chess mentor taught me there is always compensation (when you follow opening principles but lose material), you just have to find the right moves.
Game: 1. e4 e5 2. Nf3 Nc6 3. Bc4 Be7 4. d4 exd4 5. c3 dxc3 6. Qd5 Nh6
The stockfish engine gives this position 0.38 to white which means it’s drawish!! Despite Bxh6 winning the knight after castling black found the only way back to king safety but now white has issues with capture on b2 threatening to win a rook and queen a pawn, after 7 .., o-o 8. Bc1? Nb4! with long term queening threats in his future w/ pushing to c2 among other things. Engine moves suggest 9. Qh5 Nc2+! (+0.89 in blacks favor Nc2 or b5), 10 Kf1 and white has problems.
Openings are hard for humans to evaluate as good as computers can, even computers can have trouble in the openings and they still do! Deepblue’s opening book was constantly being tweaked by Grandmaster Joel Benjamin throughout the match between DeepBlue and Kasparov 1996-1997. It is usually up to the programmer to set the computer in the right direction in chess openings otherwise we get some different kinda weird and “off the beaten path” games, because of the diversity of possibilities in the early moves of chess computers can find openings unclear as well.
full video here
In early computer chess, before there was great cpu power and memory, engines without a built in opening book would play silly things like putting bishops on e3 or d3 in front of pawns on either d2 or e2, bringing the queen out on move 3, mostly unheard of in Grandmaster play even master play. Though there are exceptions where it can be good, computers were a laughing stock in chess up till the mid 90’s and master strength computer software was far and few between.
In chess, computer scientists name this the ‘horizon effect’ where the computer would only see so deep to the point where it’s evaluation cut off would miss some things that are more obvious to human players, https://en.wikipedia.org/wiki/Horizon_effect. In modern software the programmers work around this in various methods, one of them being search extensions for checks and captures.
Position evaluations, if you use a chess engine you probably want an explanation of what the numbers mean, usually in the opening you will find evaluations really close to 0.00 something like +0.38 or -0.24 aren’t really terribly different, with best play is probably a hard fight to draw for the player that doesn’t have an advantage and a hard fight to win for the player with the advantage, if the engine scores you over 0.80 in the hole you should probably find a different opening continuation since your opponent will likely have the advantage for most of the game, small advantage to white or black is negligible and means the position is definitely playable almost anything up to 1.1 is playable, after 1.78 you are playing a rough game, and your only hope is that your human opponent will make a mistake and you will capitalize on it, so you can equalize the position or better. Anything where you are evaluated 3+ in the hole is respectfully resignable. If the computer evaluates your position as +3 you have quite a winning advantage of tactic up ahead, like being 3 pawns up or a piece knight/bishop. A rook is evaluated at 5 pawns so being 5 pawns up is like being ahead a rook, or a bishop and two pawns, or a few nice winning tactics at your disposal. Being down 9 or 10 points is like being down a queen, or being down a rook and a few pawns in a generally bad position, you should resign and maybe find a new game with a more equal fitted partner unless you want to play on in hopes of a serious blunder!! Many players will play on down 5 or 6 points in hope their opponent will mishandle the game and allow stalemate by accident, this isn’t wrong! You always have the right to play on even if your opponent has calculated mate, and if he has then all the power to him. You shouldn’t force your opponent to watch your clock go down much longer than 20 minutes maybe even 10 if you should just resign, the arbiter has power to afflict you in certain ways for having bad sportsmanship. Being down 20+ to 60+ points, the computer is about to find mate in 13 and you are in a hopeless endgame, checkmate is imminent.