Learn Russian: Моя сестра напекла сладкого печенья.

The prefix на- here shows accumulation. While the standard perfective испекла simply means the action of baking was completed ("baked"), напекла specifically implies that she baked a large quantity of something ("baked a lot of").

Why is the direct object сладкого печенья in the Genitive case instead of the Accusative?

This is directly tied to the accumulation meaning of the prefix на-. When a verb uses на- to mean "producing or accumulating a large amount of something," the accumulated object must take the Genitive case. You can think of it as meaning "baked a lot of sweet cookies."

The English translation says "cookies" (plural), but сладкого печенья has singular endings. Why?

In Russian, печенье (cookie/biscuit) is almost always treated as a collective, uncountable noun, similar to how we use the word "pastry" or "candy" in English. Therefore, it stays grammatically singular (and neuter). Here, it is in the singular Genitive form. If you ever need to count individual cookies, you usually have to say "pieces of cookie" (штуки печенья).

What is the infinitive of напекла, and why is there a 'к' in the past tense?