Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
if …, then …
Then is required for the main clause of a complex sentence whose subordinate clause begins with if only when the subordinate clause is very long, as in If we get there in time and the guests have not arrived, then we can ask about tomorrow’s schedule, or when you seek greater emphasis on the result, as in If X occurs, then (and only then) will Y result.
