Two decisions, and you only have to make them once. Against 1.e4 play 1…e5. Against 1.d4 play 1…d5. That is a complete repertoire, it takes about twenty minutes to learn, and it fights for the centre on move one instead of conceding it and hoping to win it back later.
Against 1.e4
1…e5 is the honest reply. White’s most natural continuations are the Italian (2.Nf3 Nc6 3.Bc4) and the Ruy Lopez (3.Bb5), and in both of them you copy the ideas you already know from the white side: knight to c6, bishop to c5 or e7, castle, then push …d6 or …d5. The one thing you must have ready is an answer to an early queen sortie, which is why defending Scholar’s mate is the first thing I teach a new player with Black.
If you would rather not learn a dozen sharp replies, take the Caro-Kann.
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h 1 That bishop on f5 is why the Caro-Kann is easier than the French. In the French, 1…e6 2.d4 d5 leaves the same bishop stuck behind its own pawn on e6 for the next thirty moves, and learning to free it is a project.
The Scandinavian is simpler still: 1…d5 2.exd5 Qxd5 3.Nc3 Qa5 and Black has one plan, …Nf6, …c6, …Bf5, …e6, castle. It breaks the rule about the early queen, on purpose and with the tempo loss counted. You get a sound position and nothing to memorise.
Against 1.d4
1…d5, and if White plays 2.c4 you are in the Queen’s Gambit. Decline it with 2…e6 and develop: …Nf6, …Be7, castle, then solve the c8 bishop with …b6 or …dxc4 and …c5. The Slav, 2…c6, does the same job and keeps that bishop’s diagonal open.
Do not take the pawn and try to hold it. 2…dxc4 is playable when you give the pawn back for development, but 3.e3 b5 4.a4 c6 5.axb5 cxb5 6.Qf3 hits the rook on a8 down a completely empty diagonal, and Black is losing material by move six.
Why not the Sicilian yet
Because 1…c5 is the best-scoring answer to 1.e4 at master level and the most analysed opening in the game, and those two facts are connected. Your opponent needs to know one line to get an attack; you need to know eight to survive it. There is a page on the Sicilian for beginners with a setup that keeps the risk down if you want it anyway.
Pick your two moves, then spend the time you saved on not hanging pieces, which is where your Black games are actually being lost.