https://agendaweb.org/verbs/to-be-past-exercises.html
https://www.perfect-english-grammar.com/past-simple-exercise-10.html
https://www.englisch-hilfen.de/en/exercises/tenses/simple_past_mix.htm
https://learnenglish.britishcouncil.org/grammar/english-grammar-reference/past-simple
https://agendaweb.org/verbs/past_simple-exercises.html
https://www.english-4u.de/en/grammar/past-simple.htm
https://elt.oup.com/student/solutions/preint/grammar/grammar_02_012e?cc=co&selLanguage=en
https://agendaweb.org/verbs/past_simple-exercises-elementary.html
https://www.sabermas.org/2018/02/future-simple-in-english-futuro-simple.html
https://www.perfect-english-grammar.com/simple-future-exercise-1.html
https://www.perfect-english-grammar.com/simple-future-exercise-2.html
https://agendaweb.org/verbs/future-will-exercises.html
https://agendaweb.org/verbs/future-simple-exercises.html
https://www.superprof.es/apuntes/idiomas/ingles/gramatica-inglesa/verb-tense/exercises-simple-future-and-be-going-to-iii.html
https://www.ego4u.com/en/cram-up/grammar/simpre-prepro
http://www.aulafacil.com/cursos/l22530/idiomas/ingles/ingles-repaso-de-gramatica/present-vs-present-continuous
http://www.agendaweb.org/verbs/present-continuous-tense.html
http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/simcon1.htm
http://www.curso-ingles.com/aprender/cursos/nivel-basico/verbs/modal-verbs
http://esl.fis.edu/grammar/rules/modal.htm
http://www.agendaweb.org/verbs/modals-exercises.html
http://www.learnenglish-online.com/grammar/modals/tests/modaltest2.html
http://www.vitutor.com/gramatica_inglesa/verb_tense/past_simple_continuous.html
http://www.aulafacil.com/cursos/l22535/idiomas/ingles/ingles-repaso-de-gramatica/past-simple-vs-past-continuous
http://www.agendaweb.org/verbs/past-continuous-past-simple-exercises.html