Complex sentence: The immediate corollary is that if a language defines an infinite set of equivalence classes, it is "not" regular, and it is this corollary that is frequently used to prove that a language is non-regular.
Split into simpler sentences: The immediate corollary is that if a language defines an infinite set of equivalence classes, it is "not" regular. It is this corollary that is frequently used to prove that a language is non-regular.