My Top Favorites:
These are sites that I go back to over and over again!
English Page
My very favorite. It has great explanations, with really good verb tense contrast exercises.
Study Zone
https://web2.uvcs.uvic.ca/courses/elc/studyzone/grammar.htm
You'll need to navigate into "Verbs" and then "Tenses", but this is another site with not only explanations and exercises, but good verb tense contrasts.
Perfect English Grammar
https://www.perfect-english-grammar.com/verb-tenses.html
This site has three different pages for each tense: making, using and practicing. It also has some nice tense contrast explanations and exercises.
My English Pages
https://www.myenglishpages.com/site_php_files/grammar.php
Very wide variety of grammar topics with brief explanations and an exercise to an exercise for each.
ESL Lounge
https://www.esl-lounge.com/student/grammar-exercises.php
More Great Resources
Tes
English Grammar with Jennifer
https://www.youtube.com/playlist?list=PLEEA0D5FA42DB4C58
Many of you know Jennifer from her pronunciation site, but she has some very gentle, well-explained grammar videos as well.
Test English
https://www.test-english.com/grammar-points/contents/
Lots of good exercises here; when you first choose a topic, you're directed to the quiz and then can choose another tab for "Explanation". Be aware that it tends to be more British!
ESL Tower grammar games
http://www.esltower.com/games.html
Lots of online grammar games. You will need to enable Adobe Flash Player!
ESL Tower
http://www.esltower.com/grammarteachers.html
This link is the same as above, but printable grammar worksheets oriented to teachers--a wide variety of subjects.
Learn English Feel Good
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!
Azar Grammar
Even if you don't use the Azar grammar books, this site has a lot of useful extras: powerpoints, song activities, and extra handouts.
English 4 U
Many, many online exercises, graded in easy, medium or difficult.
ESL Resources
Nice links to exercises on other websites based on verb tense
English Hilfen
https://www.englisch-hilfen.de/en/exercises_list/zeitformen.htm
Lots of links to verb tenses, with contrasts. For the explanations, visit another page: https://www.englisch-hilfen.de/en/grammar_list/zeitformen.htm