3...Bg4: the pin is variation 5 of 6 in the Colle System course, a beginner opening course for White (ECO D04). It runs 24 moves and begins 1. d4 d5 2. Nf3 Nf6 3. e3 Bg4 4. Nbd2 e6 …. Black pins the f3 knight. White develops calmly, asks the bishop with h3 and still pushes the pawn to e4. In the lesson every move is explained on a board and then drilled; it is free.
Move by move
1. d4 – White takes the centre with the queen's pawn.
1... d5 – Black meets it in the centre.
2. Nf3 – The knight develops and guards d4 and e5.
2... Nf6 – Black develops too.
3. e3 – The Colle move. It opens the f1 bishop and keeps d4 solid.
3... Bg4 – Black pins the f3 knight to the queen.
4. Nbd2 – The knight develops and gets ready to support e4.
4... e6 – Black opens the way for the dark bishop.
5. c3 – The Colle triangle.
5... Bd6 – Black's bishop aims at h2.
6. Bd3 – The bishop goes to d3 and aims at h7.
6... Nbd7 – Black develops the knight and supports the e5 square.
7. h3 – White asks the bishop. If it takes on f3, White gets the two bishops.
7... Bh5 – The bishop keeps the pin.
8. O-O – White castles.
8... O-O – Black castles too.
The lesson goes on for 8 more moves, each with its explanation.
chessGM