Learn Italian: Ho comprato qualche limone e alcune carote. - I bought some lemons and some carrots.

The word qualche means 'some' or 'a few', but it has a special rule in Italian: it must always be followed by a singular noun. Even though you are talking about multiple lemons, you must say qualche limone.

Why is it alcune carote (plural) when the first part used a singular noun for 'some'?

Unlike qualche, the words alcuni and alcune must be followed by a plural noun. Because carota (carrot) is feminine, we use the feminine plural alcune to match carote. You could also say qualche carota to mean the exact same thing.

Could I use 'alcuni' for the lemons instead of 'qualche'?

Yes! Because limone is masculine, you could say alcuni limoni. Both qualche limone and alcuni limoni mean 'some lemons' and are completely interchangeable.

Why is it Ho comprato and not just Comprato?

To say 'I bought' in the past tense (passato prossimo), you need a helper verb. Here, Ho (I have) works with comprato (bought) to make Ho comprato. You cannot use the past participle comprato all by itself.