Learn Italian: La sua fidanzata era gelosa, quindi litigavano spesso. - His girlfriend was jealous, so they argued often.

While many verbs need a reciprocal pronoun to mean "each other" (like si parlano for "they talk to each other"), the verb litigare (to argue) does not. The act of arguing already implies interacting with someone else, so in Italian, you just use the verb on its own without adding si.

Why are the verbs in the imperfect tense (era, litigavano) instead of the passato prossimo?

The imperfect tense is perfect for setting the scene and describing habits. Era gelosa describes an ongoing state or characteristic in the past (she was a jealous person). Meanwhile, litigavano spesso (they argued often) describes a repeated, habitual action in the past.

Why is there a La before sua fidanzata? Doesn't sua already mean "his"?

In Italian, possessive words usually need a definite article before them, so you are literally saying "the his girlfriend". The only time you drop the article is for singular, close family members (like mia madre). Since a girlfriend (fidanzata) doesn't count as an official family member for this grammar rule, the La is required.

How do we know La sua means "His" and not "Her" girlfriend?