Also, what are the forces that act on the capillaries in Starling's forces again?
Starlings forces is talking about hydrostatic and oncotic pressures in the capillaries/interstitial fluid. The four forces are hydrostatic capillary pressure, oncotic capillary pressure, hydrostatic interstitial, and oncotic interstitial. (Oncotic = colloid osmotic) Hydrostatic is the pressure from the fluid (in this case water) while oncotic is pressure created by proteins in the blood plasma. Greater hydrostatic pressure in the capillaries leads to filtration, while water will move into the capillaries if oncotic pressure is higher, because water potential is lower.
Also to note, Starling’s Law and Starlings Forces are two different things, so if it ever asks about Starling’s Law, it refers to stroke volume and end diastolic pressure in the heart (more in; more out).