Hi privet student,
Moves 1.e4 c5 2.Nc3 Nc6 3.Bc4 e6 4.Nf3 Nf6 5.O-O a6 are solid book moves, you aimed into a good playable line for black.
Your opponents played 6.a3? instead of the master move 6.a4. How can you make use of your opponents inaccuracy?
7..g6?- You either play e6 OR g6. Both moves are rerly found together it's a deep positional topics in the sicilian. Once you play e6 in the sicilians pawn structure, d6 becomes unprotected by a fellow pawn and therefore needs the protection of the dark Bishop. Therefore in the Dragon black dosn't play e6, since he intends to play g6,Bg7.In the Najdorf, and the scheveningen, black plays e6 be7 (not g6 bg7).
12..d5? allows 13.e5 . Every time you move a pawn you have to ask your self- what am I strengthening. But most importantly, which squares am I weakening my control over. In this case the control over e5 is key.
Typical would be 12..Qc7 first controlling e5, then moving 13..d5. Look at the position can you push d5 safely after preparing it with 12..Qc7. Keep an eye out for 13.Bf4, 13.d4. how would you respond to both moves?
18..d4- Strong move, literally killing your opponents bishop for a couple of moves allowing Ne5 etc. Note the relation between good minor pieces to bad ones. In addition note the lack of defenders around the opponents king, and its unfavorable effect.
20..Nf3+! Nice:) did you see the move, before playing (19..Ne5)?
21.Kg2?- Black is now easily winning. What would you play if white played (21.Kf1)?
See you on (privet),