Questions about English usage are not as easily settled as mistakes in arithmetic. Even good writers sometimes disagree about proper usage.

Not long ago I took note of the fact that William Henry Harrison was elected president in 1840 and died in office, and that every president elected a 20-year intervals since 1840 has also died in office.

I said I didn't believes, as some do, that whoever is elected in 1980 will die in office. I added that if I were a candidate, I would not be deterred from running. "The jinx doesn't frighten me - mostly because there is no danger of it affecting me."

Robert A. Cruise of Falls Church says "it" shoudl have been the posessive "its," and adds: "As Casey Stengel used to say, 'You can look it up' - under 'gerund, use of possessive with.'"

As soon as I read Cruise's letter, I conceded mentally that he was right.

However, when I looked it up I was surprised to learn that only Fowler is adamant about the invariable use of the possessive in this construction. Other authorities permit numerous exceptions. Nevertheless, my research indicates that a careful writer would have used "its," not "it."

Last week, our obmudsman, Charles B. Seib, noted the passing of one of this era's great authorities on usage, Theodore Bernstein of the New York Times. In his column, Seib mentioned that Bernstein had corrected such errors as references to the "Marquis of Queensbury" rules of boxing. The correct spelling is "Queensberry." Not unexpectedly, this passage touched off a discussion among our copy editiors.

"The word should be Marquess , not Marquis ," one editor maintained. "Look it up in the dictionary, or in Fowler,"

So we looked it up in Webster's New World Dictionary, Second College Edition, which is now the official lower-case bible for The Washington Post's editiors.

Under the Ms, we found, "Marquess of Queensbury rules." But under the same dictionary's Qs we found," Queensberry rules, the rules for boxing formulated by the English Marquis of Queensberry." I one place it was Marquess plus the "bury" spelling for Queensberry; in the other it was Marquis with the "berry" spelling for Queensberry.

Stuff like that there makes copy editing kind of, you know, heavy, man. However, if a writer must make a choice, I would suggest Marguis of Queensberry . Most authorities indicated Marquess is, in this century, the form most used in Great Britain, but John Sholto Douglas, who was born in 1844, was the Marquis (not Marquess) of Queensberry. His title probably shoudl be written as it was during his lifetime, but I'm not prepared to fight over the point - bare knuckle or under any other rules.