If you need to check a grammar point or just want to learn more, here are some helpful free resources.
British Council, Learn English
https://learnenglish.britishcouncil.org/grammar/english-grammar-reference
This is aimed at people learning English as an additional language but its clear instructions and explanations on a range of topics can also be useful for English-first speakers.
Collins, Easy Learning English Grammar
https://grammar.collinsdictionary.com/easy-learning
This is a comprehensive guide to all aspects of grammar, broken down into basic and more advanced topics, presented in a straightforward way with helpful examples.
University of Sussex, Guide to Punctuation
https://www.sussex.ac.uk/informatics/punctuation/toc
This discusses each type of punctuation in a conversational style, giving examples of what not to do (and why) as well as explaining the more usual conventions.
English Lessons Brighton, Grammar
https://englishlessonsbrighton.co.uk/grammar/
This collection of grammar tips has been written by an English language tutor, covering a range of general and niche issues raised by students.