Learn Italian: Prendo qualche cipolla per la cena. - I am taking some onions for dinner.

This is the most important rule for qualche! Even though it means "some" or "a few", it must always be followed by a singular noun. So, you must say qualche cipolla, never qualche cipolle.

Could I use the other word for "some" (alcuni/e) in this sentence instead?

Yes, you can! However, unlike qualche, alcuni and alcune must be followed by a plural noun. Since cipolla is a feminine word, you would use alcune cipolle. Both qualche cipolla and alcune cipolle mean the exact same thing.

Why is prendo translated as "I am taking" instead of just "I take"?

The Italian present tense (prendo) translates to both "I take" and "I am taking" in English. The context here (like making a choice at a grocery store or kitchen) makes "I am taking" the most natural translation.

Why do we need the article la in per la cena? In English we just say "for dinner."