Most cleaning problems are chemistry problems wearing an apron. A stain is something bonded to a surface, a smell is something still releasing molecules, and hard water spots are minerals that soap was never going to move. Once you know which kind of problem you are holding, the fix usually costs a few dollars and lives under your sink already. That is the working idea behind this whole section: match the remedy to the mess instead of scrubbing harder.
The guides here run the full territory of a lived-in home: ink, blood, wax, rust, permanent marker and every other stain with a reputation; odors in carpets, mattresses and drains; glass, granite, grout, laminate and the other surfaces that each want their own treatment. You will find the honest version of every answer, including the unwelcome ones, like which etching is permanent and which discoloration means replacing the thing rather than fighting it.
Start with the featured guides above if you have a whole surface or room to deal with, or go straight to the article list for a specific stain or smell. When a job calls for a particular ratio of vinegar, baking soda, bleach or peroxide, the article says so exactly, along with the mixes you should never make. If a search brought you here mid-emergency, blot first, do not rub, and the right guide is one search away.