Tarrasch Defense

Black meets the Queen's Gambit by striking back in the centre with ...c5 rather than propping up d5 passively, accepting an isolated d-pawn in exchange for free piece play, the bishop pair, and open lines for the rooks. It trades long-term structural soundness for short-term activity, so the whole opening is a bet that your pieces will out-develop and out-hustle White's slower positional squeeze before the IQP becomes a real weakness.

• Accept the isolated d-pawn on purpose: it cramps White's pieces off c3/d4/e4 and gives you the bishop pair plus open c- and e-files for your rooks — don't panic and try to 'fix' it in one move.
• Race to finish development (Nf6, Be7, O-O) before touching anything else; the whole point of White's fianchetto plan (6.g3, 7.Bg2) is to meet a slow Black setup with Bg5, cxd4 and Nxd4 hitting the IQP directly, as in the main line's 9.Bg5 cxd4 10.Nxd4.
• Don't push an unsupported ...d4: without a follow-up like ...Bb4+ or pressure on b2 (the Schara/Tarrasch Gambit idea), shoving the pawn just hands White a free tempo and a target — only advance it when it wins time or cracks open lines for your pieces.
• If White grabs the pawn back with dxc5, recapture with the bishop (...Bxc5) and keep developing instead of chasing material back; opponents who instead let the knight land on a4 hitting the bishop often lose more time than the pawn was worth.
• Watch for the light-squared bishop getting shut in behind the e6/d5 pawns — get it out to g4 or e6 before playing ...Be7, since trading it off for a knight is a fine relief valve for the IQP but giving it up 'for free' just to simplify leaves you in a passive ending.
• If your opponent tries 5.e4!? (the Marshall Gambit) instead of the quiet main line, just take with ...dxe4 and finish developing — the position opens up for both sides, and giving the pawn straight back out of fear of holding it is the more common club-level mistake than actually losing it.