If clauses: rules and exercises

http://www.myenglishpages.com/site_php_files/grammar-lesson-conditionals.php#.VOr6DeaG-So

http://www.perfect-english-grammar.com/conditionals.html

http://www.ego4u.com/en/cram-up/grammar/conditional-sentences

http://www.ego4u.com/en/cram-up/tests/conditional-sentences-2

http://www.ego4u.com/en/business-english/grammar/conditional-sentences

http://www.learnenglishfeelgood.com/conditional-tenses-mixed1.html

http://www.learnenglishfeelgood.com/conditional-tenses-mixed3.html

https://englishwithatwist.com/2017/02/02/business-grammar-how-to-use-the-four-english-conditional-tenses-in-business/