How should I play after, or refute, the Tennison Gambit? I played a game as Black in which I fell victim to a gambit trap and soon resigned. Stockfish labled it as the &quot;Tennison Gambit.&quot;
[FEN &quot;&quot;]

1. Nf3 d5 2. e4 dxe4 3. Ng5 Nf6 4. d3 exd3 5. Bxd3 h6 6. Nxf7 Kxf7 7. Bg6+ Kxg6 8. Qxd8 Nc6 9. Qxc7 Nd5 10. Qg3+ Kh7 11. O-O e5 12. Qd3+

How should I play on after I accept the gambit, and how can I refute the gambit?
It looks like your position fell apart pretty early on, a symptom of poor opening theory.  Your King and Queen were exposed to attack within the first few moves, and White ruthlessly exploited that opportunity.

The first move that Stockfish classifies as a blunder (yes, there's more than one) is 5. … h6?? where you attempt to directly attack the advancing knight.  But the knight's threat is not countered by that attack, because it can just continue as planned to threaten your queen, pulling your king out of his home square in an emergency defence.

White then sacrifices his bishop to gain your queen anyway - a net material gain of 3 points if you include his lost knight.  While you then move your knight to attack the White queen, it's too late as he can simply move it away to safety.

The suggested move is 5. … Nc6 which pre-emptively protects your queen, turning the above sequence into a losing proposition for White; the game would then develop along a different line.

This is in general the better way to defend against attacks - protect the threatened squares which the attacking pieces want to move to, rather than where they currently stand.  To do this effectively you must think at least a couple of moves ahead, which takes practice.