Learn Italian: Dobbiamo comprare sia i calzini sia i berretti per l'inverno. - We must buy both socks and beanies for the winter.

Sia ... sia is an affirmative correlative conjunction. To say 'both X and Y' in Italian, you place sia before each item in the list. Interestingly, sia is actually the subjunctive form of essere (to be), so the literal feel is closer to 'be it socks, be it beanies'.

Could I say sia i calzini che i berretti instead?

Yes! You can use sia ... che exactly the same way as sia ... sia. Both mean 'both ... and' and they are completely interchangeable in this context. You will hear native speakers use both forms frequently.

Why do we need the definite article i before calzini and berretti when the English translation just says 'socks and beanies'?

Italian uses definite articles for plural nouns much more often than English, especially when referring to a general class of things. When using correlative conjunctions like sia ... sia, you still have to follow standard article rules, which means including i (the) before these plural nouns just as you normally would.

What is the difference between berretti and cappelli?

Un berretto specifically refers to a soft hat without a full brim, such as a winter beanie or a baseball cap. Un cappello is the general word for 'hat,' but it usually brings to mind a traditional hat with a full brim all the way around.