Q: What is the logic for the many meanings of the prefix "-in"? "Inhumane" means "not humane." But "invaluable" means "exceptionally above value," and "innumerable" means "too many to count." — Charlie Duncan, Potsdam, N.Y.

A: Most of the time, the prefix "in-" does mean "not," as in "inhumane," infallible" and "invalid." But sometimes "in-" can function as an intensifier; that is, it ramps up the meaning of the root verb.

So an invaluable object is so valuable that its worth can't be calculated, and items that are innumerable are so numerous that it's impossible to count them.

Likewise, "inflammable" doesn't mean "not flammable," but "flammable," and "inhabitable" and "incriminate" mean "habitable," and "criminate," respectively.

Other prefixes that we normally associate with negation can also be intensifiers. The "dis-" in "disgruntled," for instance, fortifies the meaning of its root word "grunt," so a person who is "disgruntled" grunts and grumbles a lot.

Similarly, "unthaw," "unravel" and "unloosen" actually mean the same thing as "thaw," "ravel" and "loosen."

Prefixes that are usually negative can convey more subtle meanings as well. The "dis-" in "disinterested," for instance, indicates not lack of interest, but impartiality. Likewise, "infamous" doesn't mean "not famous," but "famous for negative reasons."

Q: In my reading, I keep seeing "do" used for "due," and vice versa, especially in "overdo" and "overdue." Can you review the difference between these words? — Jane Sadler, Verona, PA.

A: An exploration of these terms is long overdue, but I don't want to overdo it.

The confusion is understandable. Not only do the homophones "due" and "do" sound exactly alike, but their meanings also overlap; both imply a sense of obligation and work. A "due date," for instance, (when something is due), could easily be "do date" (the date by which you must do something).

Even professional writers occasionally confuse "overdue" (past due, late) and "overdo" (to do too much), e.g., "Don't overdue it with too much makeup" (Los Angeles Times); "The Izod Center ... needs millions in long-overdo improvements." (Newark Star-Ledger).

How to avoid such mistakes? Just remember that "do" and "overdo" are verbs ("do the laundry, overdo the decorations"), and that "due" and "overdue" ("due date, overdue book") are adjectives.

That way, you'll due ... er, do the right thing.

Rob Kyff, a teacher and writer in West Hartford, Conn., invites your language sightings. Send your reports of misuse and abuse, as well as examples of good writing, via e-mail to [email protected] or by regular mail to Rob Kyff, Creators Syndicate, 737 3rd Street, Hermosa Beach, CA 90254.