Bonjour
4- When one of two events in the past interrupts the other one,
a) the action that does the interruptinf should be in "passé-composé" or "passé simple" if you've been learning it yet
b) The action that is being interrupted should be in "imparfait"
5- What tense should you use in French for the English (he/she) was used to (+ infinitive)
You should use the "imparfait ==> avait l'habitude de (words for words)
il avait l'habitude de + infinitif
elle avait l'habitude de + infinitif
hope this helps ☺☺☺