Every Mandarin grammar point, sorted by level, with plenty of example sentences.
Details
The reference to bookmark: each structure gets a short explanation, a formula and a stack of natural example sentences, organised A1 through C1 so you can both look things up and work through a level systematically. Free on the web with an optional paid ebook version. It is a reference, not a course — it explains what a structure means but gives you no drilling or production practice.