How can i make the grass bend when this cube hits it?
Here's the picture:
Whole process is very easy. You will only need to make your grass object as Cloth or Soft Body (with Weight Paint) then Convert Particle System to mesh and delete Particle Emitter as it will not longer be needed. Then add to your Cube Collision.
First of all you will need to choose if you want to use Cloth or Soft Body. Quick test shows me that Cloth simulation is fairly faster, but Soft Body will give you probably more options, as Plastics, Edge Stiffness and such.
Secondly I don't know why (and maybe someone can correct me) when your object is in Particle System simulation works, but Pinning/Soft Body Goal won't. Maybe it's a bug or maybe there is an explanation.
Note: Below steps could vary from your scene setup so don't take them as granted. I will point out most important settings and you could tweak them for your needs.
You will need to Weight Paint your grass strand. Red is stiff, green is... less stiff. You can experiment with this as you like to make grass really soft or hard.
Add to your object Cloth Simulation. I have used settings as you can see on the image. Most important things here are:
Nothing special here, only after setting your grass go to Modifier Tab and Convert Particles then delete Particle Emitter.