Chess Openings:

1.Alekhine's Defense

1.e4 Nf6
Alekhine's Defense is an example of a true Hypermodern opening. The Knight dares the White Pawns to start chasing it around the board. They usually oblige, creating the conditions for a classic battle against an overextended Pawn center.

2. Modern Variation
1.e4 Nf6 2.e5 Nd5 3.d4 d6 4.Nf3 Bg4

3. Scandinavian Variation

1.e4 Nf6 2.Nc3 d5

4. Positional Play in Chess: Strong and Weak Squares:

To understand pawn structure, you have to understand strong squares and weak squares. Your strong squares are those which your pieces control; your weak squares are those which your opponent's pieces control. Your strong squares are usually your opponent's weak squares, and vice versa.
Pawns, more than any other chess piece, have built-in strong and weak squares. Their strong squares are the two diagonal squares where they can capture the opponent's pieces. Their weak square is the square directly in front, which can be occupied unhesitatingly by the opponent's pieces. When the square is occupied, the Pawn can't advance.
Consider two Pawns side by side on the same rank, like the White Pawns on d4 and e4 in the diagram. The Pawn on d4 guards e5, which is the weak square for the Pawn on e4. None of Black's major or minor pieces can move to e5, blocking the Pawn on e4, without risking capture by the Pawn on d4. In the same way, the Pawn on e4 guards d5. The two Pawns cover and complement each other.
The pawn structure determines how well the Pawns work together. In this diagram, the c5 and f5 squares are marked in green. They represent strong squares for White, but weak squares for Black.

White's pieces can move to c5 and f5 without the slightest worry that they will be attacked by a Black Pawn. Both of these squares would be ideal for a White Knight or a White Bishop.
The corresponding squares on the White side of the board -- c4 and f4 -- represent no particular danger for White. If a Black piece threatens to move to c4 with strong effect, White plays b2-b3, protecting the square.
Black also has weak squares on a5 and h5. These are less important because they are further from the center, but they would require constant attention during a game if White started to invade the Black position via one of these squares.
Strong and weak squares are not determined only by the pawn structure. In the initial position, before either player has made the first move, White has a weak square at f2 and Black has a weak square at f7. These squares are weak because they are protected only by the King, who is not a fierce fighter until the later stages of the game.