Modal verbs - obligation

http://www.perfect-english-grammar.com/modal-verbs-of-obligation.html

http://www.learnenglish-online.com/grammar/modals/tests/modalobligation.html

http://learnenglish.britishcouncil.org/en/grammar-reference/modals-1

Infinitive or - ing?

https://www.englishclub.com/grammar/verbs-m_infinitive-ing.htm

http://learnenglish.britishcouncil.org/en/grammar-reference/verbs-followed-ing-or-infinitive-1

http://learnenglish.britishcouncil.org/en/grammar-reference/verbs-followed-ing-or-infinitive-2

http://learnenglish.britishcouncil.org/en/english-grammar/verbs/ing-forms

http://learnenglish.britishcouncil.org/en/english-grammar/verbs/verbs-followed-ing-clauses

http://www.perfect-english-grammar.com/gerunds-and-infinitives-verbs-1.html

http://www.perfect-english-grammar.com/gerunds-and-infinitives-exercise-1.html

Irregular verbs

http://www.englisch-hilfen.de/en/exercises_list/verbs.htm

http://www.englisch-hilfen.de/en/grammar/unreg_verben2.htm

http://www.corsi-di-inglese.eu/verbi-irregolari/elenco-verbi-irregolari-inglesi.php