Berlin Defence...

Hi!!!!....i am new in Chess World and i have a bit problem with the move 4.......Nxe4 in berlin defence as the theory says....I cant understand this move for Black,because then the white pieces have a great strategic opportunities..I am not sure but it would be ok the 4....a7-a6 to get rid of the white square bishop and the d7 pawn can be more flexible....  I would appreciate your opinion 

sure, it should be fine, it would just transpose into other ruy lines with a6 before nf6 i think.

The idea of Nxe4 is somewhat similar to the petroff defence, it eliminates the central pawn, making the structure less dynamic.
White does get some activity, but blacks position is so solid that it is very difficult for white to make anything of it.

Yes...you re right for Nxe4 but after that the move  5.Rf1-e1 black start having  many problems...

Not at all, d4 is a much better move, and is the mainline.

Here's an example:

All the lines shown here equalise very easy for black and he has nothing to fear, especially the main one I showed which I think is the trickiest line white has at his disposal, but Cox dubs the mainline in the diagram "a brutal equaliser".

Though I have never had any game go like that. 5.Re1 isn't often a prepared move, usually its just players who are unfmiliar with the berlin and treat it the same way they treat other ruy lines with nxe4.

After 4...Nxe4 5.d4, the mainline continues 5...Nd6 6.Bxc6 dxc6 7.dxe5 Nf5 8.Qxe8 Kxe8 and then White has several options.

The drawing percentage is very high because Black is already equal. Nonetheless, there remains much play in the position. In the Dortmund Chess Tournament, which Michael Adams won today with a draw against Kramnik, Adams won from this position with White in round two, and with Black in round three.

Sergey Karjakin is a leading specialist in this line, and his games are well worth going through. It is also worth looking at the Kasparov -- Kramnik games from the World Championship 2000 (all draws), and Judit Polgar's defeat of Kasparov with White. Polgar later defeated Karjakin in this line.

Victory for either player usually comes in the endgame as a consequence of outmanuevering the opponent in a complex middlegame.

The Berlin Wall, as this line is called, represents chess at its finest in the manner of Steinitz's "accumulation of small advantages." 

thank all of you for your tips.......by the way,except for Karjakin,do you know anyone known player that replys often with the berlin?

Kramnik, Aronian, Adams are a few names that come to mind

thanks

Kramnik, Carlsen, Caruana

Black uses tactics to go into an endgame, where white may have a small advantage, but in practical play it's the player who understands the position best is the player who wins.

Milorad Knezevic

Here's an interesting game that he won:

Three Berlins in today's round of the World Cup--all on the lower boards, where both players were mid-2600s, rather than the mismatches on top boards. Two draws, and one Black win. Here's the Black win: