These resources focus specifically on the Persian grammar. If you want to lookup how the ezafe-construction works, or how to build the Past Perfect Continuous, you'll find your answer here.
Grammar & Resources, University of Texas at AustinExtensive description of the grammar.
A deep dive into the Persian grammar, compiled by the Department of Middle Eastern Studies at the The University of Austin, Texas. While I feel the material at times could be organized and presented a little better, it is a very exhaustive walkthrough of all the grammatical structures.
The websites also features high-quality audio pronouncations throughout, as well as some video content designed for learners.
Ali Jahanshiri's Personal WebsiteVerb conjugator and concise, but extensive grammar resource.Other
A real gem. In addition to many other things, this contains one of the most concise, well-organized guides to Persian grammar you'll find, and a verb conjugator that works with both Persian script as well as roman transcription.
It was a great help to me when I was coding the grammar-parser of Farsi.school, and also my inspiration to start the page you are reading right now.
Ali is taking Bitcoin-donations, and if you find is site useful, I encourage you to support him.