The predominant forces acting on an element of fluid in the boundary layer over a flat plate placed in a uniform stream are viscous forces and inertia forces.

1. Viscous forces:
Viscous forces arise due to the inherent stickiness or internal friction of the fluid. In the boundary layer, the fluid molecules closest to the surface of the plate experience a no-slip condition, which means they adhere to the plate and move with it. As we move away from the surface, the velocity of the fluid increases, creating a velocity gradient within the boundary layer. This gradient in velocity results in shear stresses at the fluid-solid interface, which are responsible for the viscous forces acting on the fluid element.

2. Inertia forces:
Inertia forces are a result of the fluid's inertia or its resistance to changes in motion. When a fluid flows over a flat plate, it is subjected to a pressure gradient and a velocity gradient. As the fluid moves along the plate, it experiences an increase in velocity in the direction of flow. This increase in velocity creates a pressure drop along the plate, causing a pressure gradient. The inertia forces act in the opposite direction of the pressure gradient and are responsible for the fluid's resistance to changes in its motion.

3. Pressure forces:
While pressure forces do exist in the boundary layer, they are not the predominant forces acting on the fluid element. Pressure forces arise due to the pressure difference between the upstream and downstream ends of the plate. These forces act perpendicular to the surface of the plate and help to maintain the flow along the plate. However, the pressure forces are relatively smaller compared to the viscous and inertia forces in the boundary layer.

4. Body forces:
Body forces, such as gravity, electromagnetic forces, or magnetic forces, do not play a significant role in the boundary layer flow over a flat plate in a uniform stream. Therefore, they can be excluded as the predominant forces acting on the fluid element.

In conclusion, the viscous forces and inertia forces are the predominant forces acting on an element of fluid in the boundary layer over a flat plate placed in a uniform stream.