For Introduction Part II, click here: Classical.
For Introduction Part I, click here: Overview.White's fifth move of 5.f4 marks the Four Pawns Attack. White intends to overwhelm the Black position. The downside is that White's center is potentially overextended if Black is quick enough to take advantage. In addition to the vulnerable pawns, White is slightly behind in development. Black should aim, if the opportunity presents itself, to open the position without delay. The game typically continues 5...O-O 6.Nf3. In addition, to 6.Nf3, Black has tried 6.Be2, 6.Bd3, and 6.e5?!. More on these in future posts. Back to Nf3, there are two serious moves considered by Black in this position. The first is the mainline: A. 6...c5. The second is the more modern B. 6...Na6 (which I haven't made peace with yet).
A. 6...c5
7.d5 is the common move, closing the position temporarily (diagram right). 7...e6 (7...b5?! 8.cxb5 a6 9.a4! (9.bxa6?! Qa5 10.Bd2 Bxa6 =) 9...axb5 10.Bxb5 Ba6 11.Bd2 Bxb5) 8.Be2 (8.dxe6 fxe6 9. Bd3 Nc6=) 8...exd5, and now both 9.exd5 and 9.cxd5 are possible, with 9.cxd5 being much more common. Black now has two decent possibilities in 9...Re8 and 9...Bg4 (diagram below), both of which will be the subject of future posts.
B. 6...Na6 (diagram below left)
7. Be2 e5 8.dxe5 (8.fxe5 dxe5 9.Nxe5 c5 =) 8...dxe5 9.Nxe5 Nc6 =7.Bd3 the current fashion 7...e5 8.fxe5 dxe5 9.d5 c6 10.O-O Nc5 (Black must consider 10...Qd6) (diagram below right)
or perhaps better 7...Bg4!? 8.O-O Nd7 9.Be3 e5 and Black has not yet equalized in either of the 7.Bd3 lines
7.e5?! Nd7! 8.Be2 c5 9.exd6 exd6 
