Draw a diagram

Here's the diagram. All forces acting on the block are in red. The ones acting on the string are in blue.

What are the forces acting on the block?

• There is the ``normal force''. This force acts normal to the surface and is exerted by the plane on the block. The reason it's normal is that the plane is frictionless and can't exert forces along the direction of the plane. So it has to be normal.

• There is the weight of the object due to the force of gravity. This acts down.

• There is the force the string exerts on the mass. That force is parallel to the plane

Now how about the forces acting on the string? There are two drawn in blue. Because it's a string and its going over a frictionless wheel, the two blue forces have the same magnitude. By the way, the force the string exerts on the mass is the same as the force the mass exerts on the string by the third law.