I consider the following grammar websites to be among the best on the internet. Each of the first three listed below function as both textbook and reference guide. The last link, as its name indicates, is a comprehensive punctuation guide with a clean, easy to navigate interface. Although The Blue Book of Grammar offers a paid version in addition to the condensed edition linked below, all of the sites on this list are free. The tradeoff is the sites are not ad-free. I recommend reading the original host content of each site rather than clicking on third-party affiliate hyperlinks displayed on the various web pages. You will find each resource on this list publishes content that is rigorously vetted and provides for its readers a solid foundation on which to confidently build and develop as a writer.
The Blue Book of Grammar and Punctuation
https://www.grammarbook.com/english_rules.asp
Daily Grammar Lesson Archive
https://www.dailygrammar.com/archive.html
English Grammar dot org
https://www.englishgrammar.org/lessons/
The Punctuation Guide
https://www.thepunctuationguide.com
In addition to this short list of writer’s resources, my go-to dictionary is the Cambridge English Dictionary with the English (US) setting selected by default.
Cambridge English Dictionary