You don’t need to relearn English — you need the vocabulary to talk about it precisely, since CELTA requires you to explain language points to both learners and assessors using correct terminology. Know the parts of speech (noun, verb, adjective, adverb, preposition, conjunction) and the basic tense/aspect system (present simple vs. present continuous vs. present perfect, and their past/future equivalents). If any of these feel shaky, this is the single highest-leverage thing to review before your course starts.