There are two basic reasons why cars won't start if the starter itself is actually working: no gas, or no electricity to the plugs.
Since you have a Forester from before the "fuel filter in the tank" era, you could check the tubing into/out of the filter to see if gas is flowing. Just a very, very quick test as this could spray gas into the engine compartment if something goes wrong. A plugged fuel filter could be your culprit.
As for electricity, check for a spark coming out of the coil by holding one of the plug wires (with good insulation between you an it) up near the block while cranking. Easiest to do this after dark when visibility will be better.
2003 XS Premium MT >200k miles! ● 2005 Impreza 2.5 RS MT ● Robin-Subaru Powered Generator