Just put fenders on. Rear is fine, front brake is now staying closed whenever I use it.

What to do? Brakes in question are "Tektro R317 long-reach dual-pivot caliper brakes" according to the spec sheet for the bike.

All I did was undo the bolt in the top of the fork. Slip the brakes off. Insert fender, slip brakes back on, bolt closed. Now the brakes will close, but not open.

First see if the cable and housing are still routed correctly and freely and not kinked by the fender. Then remove the brake again and see if the arms move freely off of the bike. Also check that the return spring hasn't come off of its seat. If they do, there is something dragging or binding from the fender. If they don't check the arm clearance adjustment bolt (the 3 mm Allen bolt and 10 mm nut on the side arm) has changed it's position and adjust it until the arms move freely.

First thing I think of is that the caliper is rotated off center so one pad is rubbing all the time.

Fenders tend to inhibit the closing in of calipers, being that they reside within the caliper. Look for any contact between the fender and caliper. Disconnect the cable to eliminate that part from the picture. Try removing the fender and reinstalling the caliper, still a problem?

With no photos or our being there all we can do is guess along a list of possibilities. You're the one whom has to figure it out. I suspect an experienced eye would find the problem pretty quick. Maybe ask your LBS to help you out? Andy

Grease the cable , in the housing.

Check the spring position and tension. The low end Tektro brakes on one of my bikes demands occasional fiddling with the springs. They tend to pop off when the arms are opened for wheel removal. And the heat treatment is inferior so the spring loses tension easily and needs to be physically re-bent once in awhile. I suspect they spec low cost springs to meet a price point, so we end up fiddling with the brakes to keep them functioning.