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Clutch pedal isn't disengaging the pressure plate all the way, fairly common issue.
Rangers use a hydraulic clutch system, so if ANY air gets into the system, you lose full clutch pedal, simple as that.
Check clutch reservoir first, in engine bay, make sure it is not dry.
Then you will need a helper to push in on the clutch pedal while you open the bleeder valve on the bell housing.
Google: Bleed ford ranger clutch
Lots of videos, as said common issue
Why it is hard to change gears?
The engine is directly connected to the flywheel and Pressure plate, the Clutch disc sits in between flywheel and pressure plate.
The clutch disc is directly connected to the input shaft of the transmission
Transmission is directly connected to the rear wheels: tire, axle, driveshaft and transmission....all connected
Neutral basically disconnects input shaft and output shaft, so input shaft can spin at any RPM, but transmission(output shaft) is ALWAYS at tire RPM
So........when you are stopped, transmission is spinning at..............yes, 0 RPMs, tires aren't moving so 0 RPM
Engine is idling at 700RPM
So to put transmission into 1st gear, the Clutch disc(input shaft) MUST slow down to 0 RPMs, to match output shaft.
Clutch disc is 10" wide/across
Synchro-mesh gears are about 4" wide/across, Synchro-mesh is the softer metal "gear" that you are pushing on when you shift into any gear, it is there to MATCH RPMS so a gear can engage, without grinding.
So clutch pedal down all the way, you push on that 4" mesh to slow down that 10" clutch from 700RPMs to 0 RPMs, can't/won't go into gear until clutch disc is at 0 RPMs
So..........if there is ANY RUBBING of the clutch disc on the flywheel or pressure plate, then it gets VERY HARD for the 4" mesh to slow down that 10" disc.
If you shut off the engine then BOTH shafts are at 0 RPMs, so easy peasy to shift gears
If there is air in the hydraulic system the pressure plate doesn't move back far enough so clutch disc RUBS and can't slow down easily.
While moving the RPM matching is usually easier, but can still be hard to shift.
There is also a Pilot Bearing, it holds the end of the input shaft in the middle of the flywheel, if Pilot bearing were to seize up then input shaft would spin at the same RPM as engine, no slowing it, or clutch disc, down easily
But there is usually a noise associated with bearing issues, and you don't mention a noise