UsageLinks and learning materials about the rules governing English diction and idiom
Diction, meaning “word choice,” is the term referring to the usage of specific individual words. Errors in diction typically involve a confusion between two similar words, like allusion and illusion.
Idiom refers to the conventional usage of set expressions. Although we often think of idioms as colorful, figurative expressions (e.g., “a few tacos short of a combo plate” to describe someone who is either stupid or crazy), the term actually has a much broader meaning. Any specific, fixed combination of words traditionally used to express an idea can be considered an idiom, such as “interested in” (not “interested on”).
Usage is the general term for highly specific rules about how we use words that are determined by convention (common or traditional practice) rather than the more generally applicable rules of grammar, such as those governing verb tense or subject-verb agreement. Rules of usage often have nothing to do with logic, and in fact can sometimes be illogical and inconsistent.
My prodigious List of Common Errors in English Usage, a compendium of miscellaneous diction and idiom errors that are often tested on the SAT, ACT, and other standardized tests, is a useful reference for specific questions about usage. And here is a diction and idiom errors quiz on the expressions covered on the first two pages of that list. For a slightly different electronic version of the same quiz, see this page.