Chosen Solution
How do I get my old Kenmore sewing machine to do the regular straight stitch and forward stitches, and NOT just the stretch and reverse stitching? I have read the manual over and over again, oiled and tuned, triple checked that the special stitch modifier is in the right position, all settings are correct etc.
Sounds like the machine’s reverse button may be stuck in the ‘pressed’ position. Squirt some WD-40 or similar release oil on the button’s shaft and internals, to free it up.