I was playing against the French Defence Exchange Variation, and I was wondering if you could help out with the accuracy of my play against whites sacrifice.
You always want to be careful about any move of a pawn in the vicinity of your castled king. Here, you're right to wonder, IMO. There's no need whatsoever to worry about the threat of Bg5. The a2-g8 diagonal is going to be blocked by your c6-d5 bulwark (assuming you intend to play c6, so your knight is free to move). Which means f6, kicking the bishop doesn't present nearly as tasty a target, while also keeping enemy knights off e5 and g5. f6 is typical in a lot of French exchange lines.
7...h6 is not necessary. Why play ...Ne7? Part of it is to allow ...f6 controlling e5 and g5, assuming your king is safe. You have nothing to fear from a bishop or knight flying to g5, and can gain time kicking them back if they foolishly dare. 7...Nd7 would be more in the spirit of development than a potentially weakening, extraneous pawn move.
8...0-0 feels wrong. The h6 push just weakens the king and enables all kinds of funny sacrificial tactics like the one you faced, some of them actually working. Also, regarding your thinking: A pawn storm would actually work better precisely because of your opponent's h3 pawn. Castle long, bring rooks to bear, g5-g4 and that's your pawn lever, hitting a knight to boot. Assessing weaknesses comes with experience (of having been crushed under exactly that formation many times over.)
10...Be6 feels right, but may or may not be right. Why? 11.f4, and I'm strangely having trouble defending the primitive f4-f5-f6 lunge (with tempi!) Seriously, it looks difficult. 11...Bc5+ 12.Kh1 doesn't give a counterattack. 11...f5 or a later ...f6 weaken the light squares (h5-g6-f7 particularly.) 11...g6 12.Bxh6, 11...g5 12.fxg5. Personally, I'd have played (note: I'm not exactly sane today) 10...a6, clarifying the knight on b5. If he takes f5, knight takes (I think?) If he takes d6, queen takes. If he retreats, 11...Bd7, retreating safely.
11. Bxh6: no, it does not quite work yet, I feel. 11.f4 was the way to go? (see above)
12...Nf5: ...Kh7 looks more intuitive, since you just gave another pawn, but it looks OK since white has no attacking pieces apart from his queen. A pawnstorm looms, though, so defending may be trickier. Especially without your h-pawn, the onrushing pawns are menacing.
15. Rae1: You don't get it? Neither do I. Probably trying to bring more attacking forces in. How about 15. f4 preventing ...Bg5 and preparing a rook lift Rf3-g3.
17...a6: It is not usually advisable to play elsewhere when your king is under threat until you resolve the safety issues. Is your king safe enough? What if 18. f4 (you can tell I love that move.) 18...axb5 fxg5 with h4-h5 soon to follow. 18...Be7 19. f5 with attack. Defence is tough. Probably 17...Bh6 to regroup with ...Bg7, maybe ...Qc8 to offer a queen trade. Neutralising the queen is a must.
Moral of the story? White sacced too speculatively, and didn't attack well enough. Black can probably survive the sacrifice in any case, though. Your defensive play was good enough, although there may be some issues. Do pay attention to pawn structure (a difficult topic in itself.) And don't try to trade off to a win too hastily (12...Kh7 makes defending easier by keeping another pawn around your king.)