Clutch pedal not releasing from the floor. Intermittantly 2002 Subaru Impreza

Why would a clutch pedal stay down on the floor, car still moving ok, only after the car warms up.

I would say that your clutch master cylinder is failing.
I would suggest replacing it, along with the slave cylinder, before you wind up stranded somewhere.

When the clutch pedal sticks on the floor, is the clutch engaged or disengaged i.e. can you still accelerate the Subaru with the pedal on the floor? Describe the sequence of the pedal staying on the floor. For example, you push the pedal to the floor to engage a gear; you select the gear and engage same; you release the pedal to engage the clutch; the clutch engages; and the pedal returns to the floor.

I push the clutch in, shift gears, it stays almost all the way down. I can drive it ok but I have to pull the pedal back up with my foot and it flops up loosely and quickly. I can still drive ok.

Is it sometimes difficult to shift and is it grinding when shifting into reverse?