Suffixes: -Er Versus -Or

In my travels today, I came across a blog neighbor (just down the Interstate from me) who has much to say on grammar and language. I suspect we will clash on some topics, but I found some nice, chewy food for thought.

Dan's recent post on the word impostor made some excellent points about taking the commonality of non-standard uses (or culturally accepted uses, like flavour versus flavor) into account when mocking the “spellingly” challenged.

He also makes the point that no one has come up with a rule for remembering which words use the-or ending. Many people have attempted to write one, but the choice is based in preference and word etymology rather than something quantifiable. I prefer the -or ending and use that as a placeholder for words that I need to look up before making them public.

I can't say that I'm surprised that the -er ending has proved more popular, however. It's how children are taught to make a word for someone who performs an action: a person who advises is an adviser. (Except that he or she is not. The word is advisor.) A person who shops is called a shopper and one who hops is termed a hopper. Why wouldn't one who imposts be an imposter?

Ah, you see the problem now, don't you? Not all words ending in -er or -or have had a suffix appended, at least not to an immediately recognizable root. Can you find the poser in impostor?

Topics like these keep a blog like this going. Do you have any tips for remembering which suffix converts a verb into the noun for a person who performs that action?