Learn Hardcore Japanese: zyugyou ga hazimaru mae ni pasokon no dengen wo iremasu. - Before class starts, I turn on the computer's power.

The particle が here marks 授業 as the subject of the subordinate clause 授業が始まる (“class begins”). In Japanese, が is often used inside relative clauses to indicate what is doing or experiencing the action.

Why is 始まる in its dictionary (plain) form instead of the polite 始まります?

When you attach a time-marker like 前に to a verb, you use the plain (dictionary) form of the verb. The politeness level only applies to the main clause’s verb (入れます), not to the verb in the time expression.

What role does 前に play in the sentence?

前に means “before.” It turns the preceding clause 授業が始まる into a time expression: “before class begins.”

Why is 前 followed by に?

Particles like に mark points in time. When you say “before X,” you attach に to 前 to indicate the timing (“at that point”). Without に, 前 would just be a noun meaning “front” or “before,” not a temporal marker.

What does パソコンの電源 literally mean, and what does the の do here?

Literally, パソコンの電源 is “the power (電源) of the computer (パソコン).” The particle の connects the two nouns in a possessive/genitive relationship, so it’s equivalent to “computer’s power (supply).”

Why do we say 電源を入れます (“turn on the power”) instead of パソコンを入れます?

In Japanese you don’t directly “turn on” a device; you “turn on the power” that runs it. 電源を入れる is the standard collocation for powering up electronic devices. Saying パソコンを入れる would sound odd.

Why is the main verb 入れます in the polite form?

The speaker is using polite (ます-form) language, which is common in classroom or workplace settings, or whenever you want to be courteous.

Why is there no explicit subject like “I” in the sentence?

Japanese often omits subjects when they are understood from context. Here it’s clear that the speaker (or writer) is the one who will turn on the computer’s power, so there’s no need to say 私が.

Can you use 授業が始まるまでに instead of 前に? If so, what’s the nuance difference?

Yes, までに (“by [that time]”) can often replace 前に:
• 授業が始まるまでにパソコンの電源を入れます。
The nuance is slightly different: までに emphasizes completing the action at any point up to that deadline, while 前に simply points to the time just before the event. In many contexts they overlap.