Stains on wood floors are typically watermarks, caused by spills or over-watering plants. To get to the root of the problem, the area must be sanded, sometimes bleached, and then refinished to match the rest of the floor, which is the trickiest part. It's a good idea to hire a professional for this work, particularly if the stain is in a highly visible area.

Typically, the process is to:

1) Sand the area with a vibrating sander, first with 100-grit sandpaper and then with 150-grit sandpaper.

2) Vacuum the area.

3) Wipe the area with mineral spirits for an oil-based stain and water for a latex stain.

4) For a dark stain, apply bleach or an oxalic acid solution to lighten the stain and then lightly sand with 150-grit sandpaper.

5) Mix up one or more stains to match the existing floor color. This process usually requires testing possibilities on a piece of wood that's the same species as your floor.

If you can't find replacement pieces, cut them from a closet or another inconspicuous part of your existing floor and then replace that cutout with newer flooring.