Learn Hardcore Korean: sueobi beolsseo kkeutnasseoyo.

끝나-: <a class="border-b border-dotted border-blue-400 dark:border-blue-500 cursor-pointer text-inherit no-underline" href="/grammar/korean/verbs/stems-and-endings">verb stem meaning “end.”
-았/었-: past-tense marker. Since 끝나 ends in a vowel + ㄴ , we use 았 → 끝났- .
-어요: polite ending (present/past tense). Combined, 끝났어요 = “ended” in <a class="border-b border-dotted border-blue-400 dark:border-blue-500 cursor-pointer text-inherit no-underline" href="/grammar/korean/honorifics/jondaemal-vs-banmal">polite speech .

이 is the subject marker attached to a noun ending in a consonant. It labels 수업 (“class”) as the sentence’s subject. If the noun ends in a vowel, you’d use 가 instead. Both 이/가 mark the subject, but sometimes they also carry nuance:

이/가 often introduce new information or emphasize the subject.
은/는, by contrast, can mark topics, show contrast, or indicate known information.

What is the nuance of 벌써, and where is it usually placed in a sentence?

벌써 means “already” and expresses surprise or unexpectedness that something happened sooner than expected. Typical placement is right before the verb or the adjective:

수업이 벌써 끝났어요.
You could also place it at the very beginning for extra emphasis:
벌써 수업이 끝났어요?
That fronting intensifies your surprise.

How is 끝났어요 formed grammatically? Can you break it down?