Indian Defense

1...Nf6 is the most flexible reply to 1.d4 — instead of staking a claim in the centre right away, Black develops a piece first and waits to see White's setup before committing to a pawn structure. It's the gateway into the King's Indian, Nimzo-Indian, Queen's Indian, Grunfeld and Queen's Gambit Declined families, and at club level it's just as often met by a London-style Bf4 or an offbeat gambit as by "main line" theory.

• Keep the pawn structure flexible after 1...Nf6 — decide between ...e6, ...g6, ...d5 or ...c5 only once you see whether White answers with c4, Nf3, Nc3, or an early bishop sortie.
• Against the ever-popular London setup (Nf3 plus Bf4 without c4), strike the centre at once with ...c5 rather than developing passively — it's the standard way to question White's slow bishop move.
• Never grab the loose b2-pawn with an early ...Qxb2 after ...Qb6 — Nc3-b5 tricks routinely trap or win the queen in these structures, so treat b2 as poisoned until your king is safe and your pieces are out.
• If White plays 2.c4 instead, that's the real fork in the road: ...e6 keeps the Nimzo/Queen's Indian door open, ...g6 heads for the King's Indian or Grunfeld, and ...c5 heads for Benoni-style counterplay — pick one family and commit rather than mixing setups.
• Don't get greedy against offbeat gambit tries like 2.g4 — grabbing material you haven't studied (the Nxg4 lines) is riskier than just developing soundly with ...d5 or ...g6.
• Castle early and aim for a central break with ...c5 or ...d5 no matter which system White chooses; passive, structure-less development is the single most common mistake at this level.