Learn Italian: Ho riletto il documento, ma c'è qualcosa di strano. - I reread the document, but there is something strange.

The prefix ri- adds the meaning of doing an action again. When you add it to a verb with an irregular past participle like leggere which becomes letto, the new verb keeps that exact same irregularity. This is why the past participle of rileggere is simply riletto.

Why is there a di between qualcosa and strano?

When you follow an indefinite pronoun like qualcosa or niente with an adjective, Italian requires you to connect them using the preposition di. You cannot say qualcosa strano; it must always be qualcosa di strano.

Why does strano end in -o if qualcosa ends in -a?

Even though qualcosa ends in the letter a, it is treated as a masculine singular concept when modified by an adjective. Because of this rule, any adjective following qualcosa di will always take the masculine singular -o ending, making it strano rather than strana.

Can I attach the iterative prefix ri- to any Italian verb?

You can attach ri- to most action verbs where repeating the action makes logical sense, very much like the English prefix re-. Just be aware that while rileggere literally means to read again, some ri- verbs take on slightly different figurative meanings depending on the context.