nihongotojapan.com/en/grammar/grammar-keigo-kenson
Guide to humble language (kenjōgo) for JLPT N4, covering special verbs like 申す, 参る, and いたす, with core principles, common mistakes, and practice questions for business communication.
GrammarBusiness and keigoKeigo and politenessJLPT N4Business JapaneseJLPTSentence patterns