Learn Italian: Ebbero il dominio del mare, quindi furono forti. - They had the dominion of the sea, therefore they were strong.

They are the passato remoto (remote past) forms of avere (to have) and essere (to be) for 'they'. Avere becomes ebbero (they had), and essere becomes furono (they were). Both are highly irregular in this tense and must be memorized.

Why use the passato remoto (ebbero, furono) here instead of the imperfetto (avevano, erano)?

The passato remoto frames these facts as completed historical events in the distant past. If the writer had used the imperfetto (avevano, erano), it would sound more like they were describing the ongoing background setting of a story rather than stating a finished historical fact.

Why is the adjective forti instead of 'forto' or 'forta'?

The dictionary form of the adjective is forte (strong). In Italian, adjectives that end in -e in the singular do not have separate masculine and feminine forms. They simply change to -i in the plural. Since the subject 'they' is plural, forte becomes forti.

Why does the sentence use del mare instead of just di mare?