How do I clean rust off of a bread knife?

Use vinegar to clean the rust, leave the blade in the vinegar (the normal one, not big deal) overnight, in the morning wash it with a lil of baking soda, or if this doesn’t work (when theres a lot of rust) rub the blade with a damp piece of very fine grade emery paper, followed by rubbing it with a slice of onion, rinse well with hot water and dry it with a soft cloth.

I use Scotch-Brite pads. Those green scrubing pads

I would recommend starting with the Scotch-Brite pads, then moving on to the vinegar if the Scotch Brite didn’t do it for you. Scotch-Brite is designed to remove rust, I would be concerned that the vinegar would etch the surface, leaving it more likely to start rusting again.

If this is a stainless-steel knife, the rust is from letting it lie around next to something
else; stainless doesn’t rust nor can it be etched with something as benign as vinegar. Even if it’s carbon steel, it’s fine to use vinegar. Carbon steel is softish which makes it wonderfully easy to sharpen to a keen edge; it’s fine to let it get
etched a tiny bit, because you’re going to re-sharpen it anyway. But I would be surprised if anyone would make a carbon-steel bread knife, because it would be impossible to sharpen all those little teeth one at a time. Have you ever sharpened a chain saw blade, for example? Very, very boring.