I was on an urban light rail train recently in Charlotte, N.C., and noticed this message on the scrolling LED display. I wondered whether non-native English speakers would have trouble understanding the message.
The message is "Train is about to move." The word about in this sentence is an adverb used with the infinitive "to move" and is meant to tell riders that the train will be moving soon. It announces an intention. (Reminder: An adverb can modify a verb and tells where, when, how or to what degree.)
As R.W. Burchfield's version of "Fowler's Modern English Usage" notes, in American English, about is often expressed as a negative: "My mother was not about to let me go to the beach unchaperoned after my high school graduation."
These uses of about seem perfectly idiomatic to me as a native speaker, but I also know that about has other meanings and uses. As a copy editor, I know the word also comes with some usage baggage.
First the grammar geek stuff: About can be a preposition meaning "concerning" as in "I asked the store clerk about the dress I saw on the mannequin." It can mean "near" as in "The man who robbed the liquor store was about 6 feet tall and weighed about 200 pounds."
About is a adjective in this sentence: "The little boys were up and about at 6 a.m."
And here is an interesting angle on about from Fowler's. In a construction such as "The library will be closed for about three months during renovations," about is an "intensifying predeterminer." It is an adverb that could be replaced with another adverb such as "approximately" or "roughly."
Among journalists, the word is considered jargon as in the police report that "the victim was beaten about the head and shoulders." Copy editors usually change that to "on the head and shoulders." (I view "about the head and shoulders" affectionately because a colleague and I giggled over it regularly in my first newspaper job.)
Copy editors -- especially devotees of Theodore M. Bernstein's "The Careful Writer" -- know that about is used in approximations, so a sentence such as "The crowd was estimated at about 7,000" contains a redundancy. We've already used the verb estimated so we don't need about.
Also, as a corollary, many copy editors say the word at can be deleted in a sentence like this: "The accident happened at about 6:30 a.m." But as Bryan A. Garner writes in "Garner's Modern American Usage," "... in many contexts, especially those involving expressions of time, the phrase at about is common, idiomatic and unimpeachable." It seems to me, though, that taking out at does no harm.
Bernstein also wrote that about should be preferred over the casual around for the meaning of "approximately." I part ways with Bernstein on that. The construction "the accident happened around midnight" is fine with me.
If you'd like to read more about words that are sometimes prepositions and sometimes adverbs, check out The Tongue Untied.