I used to know a Henry who was known as H4 Stewart because of his inclination to 1.h4 as an opening move, and I'm sure it saved him a lot of time sweating over the latest line in the Sicilian Dragon. How should you reply to openings like this? If faced with 1.h4, or anything else unusual, just keep playing good chess - keep calm, keep developing, keep your eye on the centre, and keep your wits about you.
Let's put a bit of detail alongside that.
First thing is, there's usually two games going on: the chess and the kiddology. An unusual opening is a challenge. It may be a trick. Depending on the move, your opponent may be thumbing their nose at you, saying: I can beat you even with this rubbish. You aren't worth taking seriously.
Unless and until you are losing, play the board, play good chess, and play to your strengths. I'll review this and some of the other responses I have seen with examples.
Blackburne's Shilling Gambit: 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nd4 4.Nxe5? [4.c3 Nxf3+ 5.Qxf3] 4...Qg5 5.Nxf7 Qxg2 6.Rf1 Qxe4+ 7.Be2 Nf3#
The Englund-Charlick Gambit: 1.d4 e5 2.dxe5 Nc6 3.Nf3 Qe7 4.Bf4? (4.Nc3; 4.Qd5) 4...Qb4+ 5.Bd2 Qxb2 6.Bc3 Bb4 7.Qd2 Bxc3 8.Qxc3 Qc1#
Also there are numerous dodgy lines based on long diagonal cheapos:
Grob's Gambit: 1. g4 d5 2. Bg2 Bxg4?! (2...c6) 3. c4 c6 4. cxd5 cxd5 5. Qb3 and Black may have to return two Pawns.
N.B. The parallel Sokolsky line 1. b4 e5 2. Bb2 Bxb4 3. Bxe5 Nf6 is nothing like so awkward for Black, and is a good equalising try.
Look before you lunch.
Expect your opponent to play the best move next move, even if they have just played one real lemon.
The parallel mistake in attitude is:
Lasker,Em - Bauer,I (Amsterdam) [A03], 1889. 1.f4 d5 2.e3 Nf6 3.b3 e6 4.Bb2 Be7 5.Bd3 b6 6.Nf3 Bb7 7.Nc3 Nbd7 8.0-0 0-0 9.Ne2 c5 10.Ng3 Qc7 11.Ne5 Nxe5 12.Bxe5 Qc6 13.Qe2 a6 Black has done nothing to interrupt White's clockwork attack 14.Nh5 Nxh5 15.Bxh7+ Kxh7 16.Qxh5+ Kg8 17.Bxg7 this was the first time such a sacrifice was played; now, we would call this part of technique 17...Kxg7 18.Qg4+ Kh7 19.Rf3 e5 White will recover his investment with interest 20.Rh3+ Qh6 21.Rxh6+ Kxh6 22.Qd7 Bf6 23.Qxb7 Kg7 24.Rf1 Rab8 25.Qd7 Rfd8 26.Qg4+ Kf8 27.fxe5 Bg7 28.e6 Rb7 29.Qg6 f6 30.Rxf6+ Bxf6 31.Qxf6+ Ke8 32.Qh8+ Ke7 33.Qg7+ 1-0
So, against the Sokolsky Opening, I once prepared the Outflank Defence. Brodie-Regis (Thetford Major, 1979) went 1.b4 Now 1...c6 with three ideas: ...a5, ...Qb6 or simply ...e5. 2.Bb2 a5! [2...Qb6?! 3.a3 a5 4.c4 axb4 5.c5!] 3.a3 axb4 4.axb4 Rxa1 5.Bxa1 Qb6 6.Bc3? Na6 and I was already winning a Pawn: 7. e3 Nxb4-+. If 7.Qc1 Nxb4! 8.Qb2 Nd5 9.Bxg7? Bxg7 10.Qxg7 Qxb1# Some analysis goes 6.c3 d5 7.Nf3 Bf5 8.d3= [Not 8.g3? Qa7 9.Nh4 Be4 10.f3 Bxf3 11.Nxf3 Qxa1]. However, in the following round Osland-Regis (Thetford Major, 1979) went 2. e3! (the theoretically recommended reply) and I was already frowning.
[I do like structural advantages, so I also prepared 1. g4 e5 2. Bg2 h5! Similarly, a better try for White is 2.d3!]
Let's have a look at how you can go about busting 1. e4 e5 2. Nf3 f6 Damiano's Defence. We can say safely that this is unsound, but can you prove it?
OK, start 3.Nxe5 fxe5 4.Qh5+ Ke7 (not 4...g6 5.Qxe5+ Qe7 6.Qxh8) 5.Qxe5+ Kf7 6.Bc4+ d5 7.Bxd5+ Kg6 winning easily. Next move? Lots of folk have played 8.d4?? Bd6. Try instead 8.h4 h6 9.Bxb7 Bd6 (9...Bxb7 10.Qf5#) 10.Qa5 Nc6 11.Bxc6 Rb8 which becomes the Five Pawn Gambit(!) if you fancy 12.Qxa7.
So, well done, you've won that game, but next round your opponent goes 3...Qe7. Now think for goodness' sake, and don't snap out 4.Qh5+?? g6 5.Nxg6 Qxe4+ 6.Be2 Qxg6! Be satisfied with a small advantage after 4. Nf3. So, I think you can play 3. Nxe5 with confidence: there are two traps but they are both pretty straightforward. If you are confident of working out the tactics at the board, them all you need do is remember (1) 3. Nxe5 is playable, (2) there are two traps.
So, this is OK if you can do your homework, but keep your powder dry - brush up on it occasionally. Also, you must accept that, unless you are also learning some cute tactical ideas, most of your time is going to be pretty well wasted, so keep the time spent and the memorising done down to a minimum. Since adopting this approach I have lost one game to an unorthodox opening, and been in trouble against another, but have won every other game.
There are some very hairy gambits which you don't need to refute, but instead you can play conservatively and expect to outplay your opponent thereafter. So Regis-Bax, 1982 went 1. e4 e5 2. Nf3 f5. Now Black was all ready to snap out his analysis after the critical tactical line 3. Bc4 fxe4 4. Nxe5 Qg5 but he visibly slumped after I played a positional line of Nimzovitch's: 3. Nxe5 Qf6 (Nimzo played 4. Nc4 and eventually d2-d3) 4. d4 d6 5. Nc4 fxe4. [Although this line isn't very sharp it is crucial for the success of the Gambit. So it is still "theory", but I knew the ideas and he didn't.]
The exceptions are pretty obvious: you can't nonchalantly play ...c7-c5 after b2-b4, and ...Nf6 may walk into g2-g4-g5. But you can rehearse your favourite system against most dodgy stuff from your
Oddly, if your opponent is trying to stop you trotting out 10 moves of a standard opening by playing something odd like 1.a4, it may be that your best response is to do exactly that!
For example, how do you respond to 1. a4? It depends on what you play ordinarily, but try 1...e5. Now, in most lines of standard defences to 1.e4, Back never plays 1...a5 and often wants to play 1...a6 - think about the Ruy Lopez and Sicilian Defence. The only line of the Reversed Ruy I can think of where you can make a virtue of 1. a4 is the cheapo 1.a4 e5 2.e4 Nf6 3.Nc3 Bb4 4.Na2?! hoping for4...Ba5 (4...Be7) 5.b4 Bb6 6.a5 Bd4 7.c3, although I wouldn't be ready to resign just yet after 7...Bxf2+ 8.Kxf2 Nxe4+! The only defence I can think of where Black actually want to play 1...a5 is a line of the French Advance (1. e4 e6 2. d4 d5 3. e5 c5 4. c3 Nc6 5. Nf3 Qb6 6. a3 a5, and even then it's not the best move!), so as long as you don't insist on a reversed version of that you should be OK.
But see what we are doing here: we are taking the exact position in front of us seriously, we are weighing up the advantages and disadvantages of our opponent's moves, and we are playing chess and not kiddology. (My Dad has this lovely term persaudo-intellectual, which is how he pronounces pseudo.)