Good General Sites when you need to look for something!
https://www.englishpage.com/index.html
Great explanations with exercises.
Grammar Hilfen
https://www.englisch-hilfen.de/en/grammar_list/alle.htm
Yes, it is a German-based site, but with great resources--explanations and exercises
English Zone
Very abbreviated lessons; most with exercises.
Using English
https://www.usingenglish.com/quizzes/
Lots of exercises on a variety of topics
Learn English Feel Good
http://www.learnenglishfeelgood.com/esl-english-grammar-exercises.html
OnlinePrintable
This website may not have explanations, but such a wealth of exercises--great for when your students need extra practice--great upper level resource!
https://www.learnenglishfeelgood.com/esl-printables-worksheets.htmld.com/esl-printables-worksheets.html
World English
Not very colorful, but each page has a short explanation with several quizzes at ascending levels.
EGrammar
Not the flashiest site, nor the most complete explanations, but the exercises are very good.
Guide to Grammar
https://guidetogrammar.org/grammar/
Go to the "Word and Sentence Level" for grammar topics.
Road to Grammar
You have to scroll through an alphabet list, but you'll have a lesson and quiz.
Voice of America Everyday Grammar TV
https://learningenglish.voanews.com/z/4716
Yes, Voice of America has grammar video lessons, and they're quite good! No exercises, though.
Specific Topics:
Passive
Lots of varied worksheets to introduce and practice
Too and Very
Exercises
Few and Little
Exercises
Subject-Verb Agreement
Explanation and exercises