An object is something small enough that you can pick it up or move it. It is a single thing, not a pile of many things. You would not call salt an object unless it was a single crystal of salt. A substance is made out of a single kind of material. It does not matter whether it is a single thing, or many things together. Salt is a good example. An article is an object of a particular kind: "The truck was full of household articles." An article is often a piece of clothing. People say "article of clothing". Also, an article is a story or report in a newspaper or magazine.