I have a Schengen visa issued by Spain and will be travelling this month. This is a single entry visa. I plan to do a short trip to France for a day and return to Spain. Am I allowed to do this with a single-entry visa?
Traveling from Spain to France or from France to Spain does not count as an "entry" into the Schengen area. You can therefore make this trip unless your visa is a visa of "limited territorial validity." This will be indicated in the "valid for" field of your visa, which will normally say "ESTADOS SCHENGEN," meaning that it is not territorially limited.
If your visa says "estados Schengen," you can certainly travel to France. If it says something other than that, you need to determine whether the visa's territorial validity includes France. For example, it might just say "España," in which case it is not valid in France (or anywhere else outside Spain). It might also list several countries by abbreviation, in which case you need to see whether France is listed, and whether the listing indicates that France is included or excluded in the visa's territorial validity.