Learn Italian: Ognuno di loro ha una responsabilità importante. - Each one of them has an important responsibility.

Yes, absolutely! In this context, ognuno and ciascuno mean exactly the same thing (each one). You can say ciascuno di loro with no change in meaning.

Since loro means 'them', why is the verb singular (ha) instead of plural (hanno)?

The subject of the sentence is ognuno (each one), which is always singular. Just like in English where we say 'each of them has' (not 'have'), Italian matches the verb to the singular pronoun ognuno.

Why do we use the pronoun loro after di here?

After prepositions like di, a, con, or per, Italian uses what are called 'tonic' or strong pronouns. For 'them', the correct tonic pronoun is loro.

I see an accent mark on the end of responsabilità. Does that change how it behaves in the plural?

Yes! Nouns that end in an accented vowel, like responsabilità, are invariable. That means their ending doesn't change in the plural. 'One responsibility' is una responsabilità, and 'two responsibilities' is due responsabilità. You only change the article or adjective to show it is plural (like le responsabilità).

Responsabilità is feminine. Why doesn't the adjective importante end in -a (like importanta)?