So past tense would be 'Uaireanta, rith sé abhaile'? Is this sentence in this section (present tense question in lesson on past tense) to catch us out, by accident or to demonstrate a particular point?
It sounds odd for me to use rith there. I'd say ritheadh sé would be better, since it's habitual. (Sometimes, he used to run home, which sounds odd to me in English). Still, however, I'm not sure how good that is in Irish.
I agree that “sometimes he ran home” would be more likely in English than “sometimes he used to run home”, but using the imperfect seems like the right choice in Irish — the entry for “sometimes” in the NEID has bhíodh sé mall uaireanta for “he was sometimes late”.