Learn Hardcore Japanese: zyugyou ga hazimarumae ni watasi ha kyousitu no seki ni suwatte hon wo yomimasu. - Before class starts, I sit in my seat in the classroom and read a book.

Because 授業 is the grammatical subject of the intransitive verb 始まる (“to begin”). The particle が marks that subject. If you used は instead, you’d be putting extra topical or contrastive emphasis on “the class,” which isn’t needed here—you simply want to say “before the class begins.”

Why do we attach に to 前 in 始まる前に?

前に is a fixed temporal expression meaning “before ….” To make “before” into a time‐marking adverb, you add に. Without に, 始まる前 is merely a noun phrase (“the time before it begins”) and can’t function grammatically as a time adverbial.

Why is 始まる in the dictionary (plain non-past) form rather than the past or –て form before 前に?

When you want to express “before doing X,” you attach 前に to the plain non-past or plain past form of the verb. The non-past (始まる前に) means “before it begins,” which fits a future or habitual context. The past form (始まった前に) would awkwardly mean “before it had begun.” The –て form isn’t used with 前に.

I see two に particles—one after 前 and one after 席. Aren’t they the same?

They serve different roles.
• 前に marks a time: “before X.”
• 席に marks a location: “at/in a seat.”
Japanese uses に both for time‐points and for locations of intransitive actions.

Do we really need 私は in this sentence?

私は marks “as for me” (the topic). It clarifies who is doing the actions, which is helpful in written or formal Japanese. In casual speech, if context is clear, you can omit it:
授業が始まる前に教室の席に座って本を読みます。

What does the の in 教室の席 do?

Particle の links two nouns: it turns 教室 and 席 into “the seat of the classroom” (i.e. “a seat in the classroom”). Without の, you can’t properly express that relationship.

Why is 座る followed by に instead of を?

座る is an intransitive verb (“to sit”), so it doesn’t take a direct object (no を). You mark the place where someone sits with に (or sometimes で). Hence 席に座る = “sit down in/at the seat.”

Why do we use the –て form (座って) to connect to 本を読みます?

The –て form connects two actions by the same subject in sequence or simultaneity. 座って本を読みます means “I sit down and then read a book.” It’s more concise than making two separate sentences.

Why is the final verb 読みます in the non-past polite form rather than past or progressive?

Here it describes a habitual or routine action (“I (usually) do this every time before class”). The non-past polite (読みます) covers both present and habitual contexts. If you wanted to emphasize that you are in the middle of reading right now, you’d use the –ている form: 読んでいます.

Could I replace 教室の席に with 教室で?

Yes, 教室で座って本を読みます is grammatical and means “I sit and read in the classroom.” But it’s more general: you’re anywhere in the classroom. 教室の席に specifically points to “in my seat in the classroom,” adding precision.