One of the biggies, here: fewer or less? Many people, including a good friend of mine, get absolutely incensed when these are used incorrectly (leading to the spectacle of several well-educated, intelligent ladies self-correcting nervously when they make a slip in front of her: she probably doesn’t warrant this fear any more than I warrant the fear people apparently have of making a typo in a response to this blog!)
Anyway, it’s quite simple …
Fewer is used for countables. Less is used for mass nouns denoting things that can’t be counted – uncountables.
So – it should be “5 items or fewer” on that supermarket checkout sign. Less coinage has been produced this year, but there are fewer coins. Less hair has been cut off this time, but I have fewer grey hairs than he does.
The Concise Oxford English Dictionary is reassuringly stern on this one: “The use of less with a count noun (“less people”) is incorrect in standard English.” That’s told you!
You can find more troublesome pairs here.