John in Albany, N.Y., had a question about something he heard in a TV commercial: "Dentures are different to real teeth."

The preposition struck him as odd. "I don't recall ever hearing this [different to] before," John wrote. "Then last week I was reading a book [written in 2006 by an English author], in which he uses the same phrase. Is this correct? I always thought 'different from' was correct ... not 'different to.' I know things have changed since my schooling in the '40s and '50s, and some things we thought were wrong are now accepted [perhaps due to repeated incorrect usage], so maybe I missed something."

In fact, John did miss something. Or, more precisely, his teachers back in the '40s and '50s missed something. It's the same thing my teachers missed in the '70s and '80s and, most likely, teachers of today miss too. The thing we all missed wasn't a lesson on what word should follow "different." It was a lesson on how to know which preposition goes with any particular word.

For example: Are you embarrassed by something? Or embarrassed of it? Do you dissociate from someone or dissociate with him? Do you direct your attention at the speaker or do you direct your attention to him? Do you have an affinity for someone or an affinity with him? Are you bored by someone? Bored with him? Bored of him?

Questions like these cause more than their share of confusion. And they raise a larger question — the question that, unfortunately, hasn't come up in many classrooms since at least before John's school days: How can you know? When you're not sure which preposition pairs with any particular verb, adjective or noun, where can you turn for a clear, authoritative answer on all that's right and wrong?

Nowhere, that's where. The lesson that none of us seem to get is that, in English, some things just are. They don't conform to rules of grammar. Expert syntactical analysis won't help you. Some things we say a certain way simply because people before us said them that way.

We call this idiom, and questions like "different from" vs. "different to" fall into this category.

"The proper preposition is a matter of idiom," wrote Theodore Bernstein in "The Careful Writer."

Sometimes, Bernstein pointed out, these choices "come naturally." But this doesn't make for a great defense in a grammar argument. I have no doubt that "different to" sounded natural to whoever wrote the copy for that TV commercial, though it sure doesn't sound good to me.

When you don't have a good sense of which preposition to use, or when you want validation of your own opinion, you can try a dictionary. In some cases, a dictionary will show which prepositions go with a certain word. Usually, these clues are in the examples. Read the whole entry for "affinity" at Merriam-Webster's online and you'll see usage examples containing both "affinity for" and "affinity with," indicating that the dictionary recognizes both forms. If you read the entry for "differ," you'll see examples supporting "differ from," "differ in" and "differ on." But the dictionary's help with idioms is hit-or-miss: Does the absence of an example using "differ with" indicate that Merriam-Webster's frowns on this formation? Not necessarily.

So what do you do when the dictionary lets you down? Bernstein has a suggestion: "The only thing to do is to consult three knowing friends and get a consensus."

That won't give you enough ammo to say someone else is wrong. But it will give you the confidence to know that a choice you made is right, kind of.

JUNE CASAGRANDE is the author of "It Was the Best of Sentences, It Was the Worst of Sentences." She can be reached at JuneTCN@aol.com.