A Question

Hello i have a Question. What should my defenses  against D4 be? The only defense i know is the dutch defense which i've only had the chance to play once. but what else should i try? My opponents don't play D4 very often but i would like a back up plan in case they do. Thanks for your answeres!

If you're not looking to play something interesting such as the Dutch, the Benoni or the KID, then the Slav is a good choice. (1.d4 d5, and if 2.c4, then 2...c6.) It's a principal move that doesn't lock in your bishop. Just mind the checks on a4/b5 if applicable.   

You can try to spice things up by threatening to take on c4, like in the semi-Slav. 

Also there is nothing wrong with the Queen's gambit or the Nimzo-Indian, but I consider these more difficult. You can look a little cramped with black and you need a counter punch in the centre at some point. If you're not ready for that then don't bother. 

It sounds like he doesn't want something dense in theory.  If that's true, than the Orthodox QGD is the way to go.

1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Bg5 Be7 5.e3 Nbd7 6.Nf3 O-O where now:

7.Rc1 is answered with 7...c6.  Black will try to make productive waiting moves until White either exchanges on d5 or moves his Bishop so that Black can gain a tempo with ...dxc4 after that.  DO NOT TAKE ON c4 if White's Bishop is still on f1.  Black will then try for the e5 pawn break.

7.Qc2 White plans to put the Rook on d1, either via Rd1 or O-O-O, and attack the Kingside.  Black should answer this with 7...c5 instead of 7...c6 in order to create counterplay on the Queensite while White does a hack job on the Black King.

Other moves are just weak.  For example, 7.Bd3 already surrenders the battle for the tempo.

I play 7.Qc2 myself as White.  7.Rc1 is the most popular, along with the Exchange Variation (4.cxd5 instead of 4.Bg5)