Learn Russian: Купи ботинки на зиму.

No, even though we wear them in pairs and usually talk about them in the plural, a single boot is ботинок (masculine). Because it is inanimate, the accusative plural ботинки in this sentence looks exactly like the nominative plural.

Why do we use на зиму instead of для зимы to say "for the winter"?

While для plus the genitive case usually means "for the benefit of" or "designed for," we use на plus the accusative case to indicate an intended time period. You are buying the boots to wear for the duration of that specific season.

Is the word stress on на зиму different from just saying зиму?

Yes! In this specific phrase, the stress shifts onto the preposition, making it sound like нА зиму. This is a traditional stress shift that happens with a few common time and purpose expressions in Russian, such as нА ночь (for the night).

Why is the perfective imperative купи used here rather than the imperfective покупай?

The speaker is giving a specific, one-time instruction to achieve a result. We use the perfective aspect for a single, complete action. If it were a general habit or recurring advice (like "always buy boots in advance"), you would use the imperfective покупай.