The best response to the Englund Gambit ($1. d4$ $e5$) is to accept the gambit and enter the Main Line Refutation ($4. Bf4$).
This line is objectively the strongest because it allows you to keep the extra pawn while Black's position collapses due to a lack of development and a misplaced Queen.
The Refutation Sequence
Memorize this exact sequence to punish the opening:
$1. d4$ $e5$ (The Englund Gambit)
$2. dxe5$ $Nc6$
$3. Nf3$ $Qe7$ (Black attacks the pawn for the second time)
Note: Black will now try their main trick with $4... Qb4+$.
$4... Qb4+$ $5. Bd2$ $Qxb2$ (Black grabs the b2 pawn and attacks the Rook.)
$6. Nc3!$ (This is the winning move. Do not play $6. Bc3??$ which loses immediately to $Bb4$.)
Why This Wins
By playing $6. Nc3$, you defend the Rook on a1 indirectly (if Black takes the Rook, you trap their Queen). You are now threatening to play $Rb1$ to harass the Queen and $Nb5$ to fork Black's King and Rook.
Black is usually forced to retreat or play awkward defensive moves while you have a massive lead in development and an extra center pawn.
The One Trap to Avoid
The only way White loses this position is by playing $6. Bc3??$.
If you play $6. Bc3$, Black plays $6... Bb4!$, pinning your Bishop. You will lose your piece and the game.
Remember: Always play the Knight to c3 ($6. Nc3$) to protect your Rook.
Relevant Video
This video is relevant because Grandmaster Naroditsky details the exact $4. Bf4$ refutation and explains the tactical nuances of the $Nc3$ vs $Bc3$ distinction to ensure you convert the advantage.
My addendum: here another video by Remote Chess Academy that deals with the gambit.
