Show HN: Chinese sentence checker with pinyin and English explanations

SentenceCOACH is a free Chinese grammar checker for Mandarin learners. Users enter a sentence and can choose an HSK level; the tool returns minimal corrections that aim to preserve the original meaning, with explanations in plain English, pinyin, and an English translation. If a sentence is already correct, it says so and may offer a more natural alternative. It accepts simplified and traditional characters and works without an account, subject to a fair-use hourly limit. The checker is intended not just to fix errors but to help learners understand them and practice producing sentences independently.

Its coverage includes word order, aspect particles and complements, commonly confused words and particles, measure words, sentence structures such as 把 and 被, and incorrect characters or word choices. The guidance recommends writing without a translator, studying the explanation, and then rewriting the corrected sentence from memory; setting an HSK level can also help flag vocabulary that is too advanced. Feedback combines a large language model with rules targeting learner errors, so it can catch many common problems but is not guaranteed to be right. It is best treated as an initial tutoring aid rather than an authoritative grade, and important work should be checked with a teacher.