http://www.engvid.com/basic-english-grammar-auxiliary-verb/
http://www.engvid.com/auxiliary-verbs-in-conversations/
https://www.ego4u.com/en/cram-up/grammar/auxiliary-verbs
https://www.tesol-direct.com/guide-to-english-grammar/auxiliary-verbs
http://grammar.yourdictionary.com/parts-of-speech/verbs/auxiliary-verb.html