1. https://agendaweb.org/verbs/mixed-tenses.html
2. https://www.english-grammar.at/online_exercises/tenses/tenses_index.htm
3.https://test-english.com/grammar-points/b1/review-verb-tenses-b1/
4. https://sites.google.com/site/meyenglishclassb2/grammar/tense-revision
