Lesson
White to move. Main line: 1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. Nf3 O-O 6. Be2 e5 7. O-O Nc6
White occupies the center with pawns on d4 and e4, while Black develops behind the flexible ...d6 structure. The move ...e5 challenges the center only after Black has castled, so opening lines cannot expose the king.
If White closes with d5, the plans split by wing: White expands with b4 and c5 on the queenside, while Black seeks ...f5 and a kingside attack. If the center opens instead, piece activity and control of d5 become more important than the pawn race.
The opening rewards understanding of pawn breaks. Black should not launch ...f5 before development is ready, and White should not expand on the queenside while the center can collapse. The position after 7...Nc6 is the decision point from which those plans begin.