1. e4 e5 2. f4 (2...c6?) why should u do it?

cant undesrtand what are the best moves after black plays 2..c6? why?

what type of opening is this? Does it transposes in sth?

It will quite likely transpose into 1. e4 e5 2. f4 d5 3. exd5 c6 which is a legit line.

That's an eccentric variation of KGD / King's Gambit Declined. U shouldn't do it, though it's playable.

The King's Gambit...

https://www.chess.com/blog/RussBell/the-kings-gambit

It generally is not a good idea to make a slow move like this against a gambit, which tends to lead to sharp open positions. There are many good 2nd and 3rd moves for black against the Kings Gambit. The only reason to play this is that white will probably have not seen it and will not be prepared for it.

Which pawn you're talking about?

Show and tell.

If white plays fxe5, qh4+ wins for black on the spot

But then Nf3 traps the queen

I mean... Does it really trap the queen? While I'm not some amazing player, please bear with here...

Yes the queen can't get out but your Bishop, Knight, and h2 pawn can't really move either. Bishop is pinned, knight is blocking the queen, h pawn, I mean moving it is just bad. I don't see how h2 or h3 accomplishes anything beneficial.

In addition, you are not attacking the queen. You are just making all possible queen moves sack the queen. But black doesn't have to move the queen.

So that brings up the question how do you attack the queen? Maybe I'm missing something, but the only options I see is 1: move the king to row 2, bishop g2, 2: queen trade.

Queen trade... You're losing still losing, don't think we need to talk about that.

Moving the king for bishop g2... I mean that's a 3-move process. d pawn move or queen f2, move the king to the row 2, bishop g2.

While you are doing that, you are not developing pieces. Black can do things like Bb4 if you move your d pawn.

Maybe f6 Ba6, protected by the knight, attacks the queen and bishop.

Maybe push d pawn and then Bh3. The queen pins white's bishop to the king, so black is not sacking a bishop. This could devolve into trading both queens and white squared bishops. Which is fine for black - Black is still up an entire rook.

Basically, yes you can prevent the queen escaping for a while but... Black has options and you are still down a rook.

Even if you manage to win the queen, you'll probably have to sacrifice your bishop. So now you've traded a rook and a bishop for a queen, +1 value. You also are far behind in development though, so you are probably still losing.

Assuming black continues to play well, I only see a losing position. Maybe a higher rated player can find a way make it a winning position.

i was going to say basically the same thing but you beat me to it. theres not really a way to attack the queen on h1 in that position. Black's still completely winning and you're disregarding the fact that while white goes to attack the "trapped" queen, black will make his own moves to improve his position

How? Black can play for example 6.Nf3 d5 7.exd6+ e.p. Kd8, when either ...Bg4 or ...Bh3 will end the game.

It's a trap. White does not want to play fxe5 in any variation of the KGD until he plays Nf3, because Qh5+ close to wins it.

thank you all for the comments, i have learned a lot..

so after 2...c6, should I just play Nf3 or d3? 

I would play 3.Nf3 and "normal chess".

I don't think white should expect any real advantage after the odd looking 2...c6, but it does not pose any threat to white's setup, either. Black will play sooner rather than later ...d5 and the game will start.

Why not 3. d4!? It looks like a funny line :

I don't see any fun here, sorry for that.

3.d4?! exd4 4.Qxd4 d5 looks like 1.e4 e5 2.d4 exd4 3.c3 d5- Danish Gambit declined in reverse, but with white here having an extra tempo. However, that extra tempo (pawn to f4) is bad for white, because he cannot plant a bishop at g5.