-um is a “non-precise” suffix that broadens or gives a different meaning to the root. Generally, you have to learn each um-construction as a separate word. And while some new um-words are occasionally created (like retumi, “to browse the internet”), it’s not really an actively productive suffix to the same degree that others are.
However, some um-words fit into categories. For example, some of them name articles of clothing:
- kalkanumo = heel (of a shoe)
- kolumo = collar (of a garment)
- nazumo = pince-nez (old-fashioned eyeglasses)
- plandumo = sole (of a shoe)
Some indicate actions done using a thing:
- akvumi = to water, to irrigate
- aerumi = to air out
- literumi = to spell using letters
- sapumi = to soap up
Some indicate actions involving a thing:
- foliumi = to leaf through (a book, etc.)
- brakumi = to hug
- okulumi = to make eyes at, to ogle
Lots of um-words, however, don’t fit in a category. Like laktumo (“milt,” based on lakto because of the milky appearance) and cerbumi (“to wrack one’s brain”).
PMEG (Plena Manlibro de Esperanta Gramatiko) has a fairly complete listing of um-words in current use.