When Black opens with Nf6 after e4 - Beginner question I would like to have some analysis in the following setting:
1. e4 Nf6 2. e5 Nd5.

Now, what is best for white? The move 3. c4 continues the attack, but can create a "pawn weakness".
1.e4 Nf6 is called Alekhine Defence.
It's an hypermodern opening as it doesn't directly fight for the center but more aim to provoke weakness in White camp and then counter-attack.

Also White has several option, first he can try not to push by playing :
2.Nc3
2.Bc4 as after 2...Nxe4 White get back the pawn with 3.Bxf7 Kxf7 4.Qh5+ g6 5.Qd5+

Still, it's better to take the center :
2.e5 Nd5 and now White has to decide whether or not he will push his pawns, so 3 systems are relevant :


the modern approach, White delays c4 : 3.d4 d6 4.Nf3 just finishing his development without giving weakness
3.d4 d6 4.c4 Nb6 5.exd6 and now Black has a choice between ...exd6 or ...cxd6 depends where he wants to put his bishop f8.
the most agressiv : 3.d4 d6 4.c4 Nb6 5.f4 just taking as much space as possible, I think this is the most critical continuation.