Learn Hardcore Japanese: doukyuusei no hitori ha hugoukaku datta node, minna de issyo ni hukusyuusiyou to yakusokusita. - One of my classmates failed, so we promised to review together.

Here の is the same の you see in phrases like 日本の車 (“cars of Japan” = Japanese cars). It links a group (同級生) to an element taken from that group (一人).

You can use the same pattern with other groups:

友だちの一人 – one of my friends
生徒の一人 – one of the students

Why is it 同級生の一人は and not 同級生の一人が? What’s the nuance of は here?

Both are grammatically possible, but the nuance changes:

同級生の一人が不合格だったので …
– “One of my classmates failed, so …”
– が presents “one classmate” as the grammatical subject and as new information.
同級生の一人は不合格だったので …
– “As for one of my classmates, (he/she) failed, so …”
– は makes it the topic, often with a contrastive feel:
“One of my classmates (unlike the others) failed, so …”

Here, は subtly implies: “Most people passed, but this one classmate didn’t; because of that, we did X.” That contrast fits naturally with the idea that everyone else passed and only one didn’t.

What is 不合格だった grammatically, and why not just use a verb like 落ちた?

不合格だった is:

不合格 – a noun/na-adjective meaning “failure / not passing (an exam, selection, test)”
だった – the plain past of だ (“to be”)

So 不合格だった = “was a failure / did not pass (the exam).”

You could also say:

試験に落ちた – “(someone) failed the exam” (more colloquial, verb)
不合格になった – “became a failure / ended up failing” (verb phrase from the noun)

不合格だった sounds a bit more result-focused and neutral, like reporting an exam result on paper. 落ちた feels more everyday and emotional (“I failed”).

What’s the difference between ので and から in 不合格だったので?

Both ので and から can mean “because,” but:

ので sounds a bit softer, more formal, and more objective.
から is more casual and direct, and can feel more like the speaker’s personal reasoning.

Here:

不合格だったので、みんなで…約束した。
– “Because he/she failed, we all promised to review together.”
– Sounds neutral and slightly polite.

You could say 不合格だったから instead in casual speech. In written style or polite speech, ので is often preferred.

What does みんなで mean, and how is it different from みんなと?

みんなで marks the group as the doer of the action:

みんなで復習する – “We all review (together).”
→ The subject is the whole group; the group is acting as a unit.

みんなと means “with everyone,” and usually implies I (or some other subject) am doing something together with everyone:

みんなと復習する – “I review with everyone.”

So:

みんなで復習した – “We (all) reviewed.”
みんなと復習した – “I (or some other subject) reviewed with everyone.”

In this sentence, みんなで fits well because the group (including the speaker) is the one that will do the reviewing.

Isn’t みんなで一緒に redundant? Why use both?

Yes, in a sense it does repeat the “together” idea, but this kind of redundancy is very natural in Japanese.

みんなで already implies “as a group / all together.”
一緒に explicitly means “together, in company with each other.”

Common patterns:

みんなで一緒に遊んだ。 – We all played together.
家族みんなで一緒に食事した。 – The whole family ate together.

You could say just:

みんなで復習しよう。
一緒に復習しよう。

Both are fine. Using みんなで一緒に just emphasizes the group feeling: everyone, together, as a unit.

What form is 復習しよう, and how is it different from 復習しましょう?

復習しよう is the plain volitional form of 復習する (“to review”):

Verb in dictionary form: する
Volitional (let’s / shall we / I’ll ~): しよう

So 復習しよう ≈ “Let’s review” / “We will review, right?”

Difference from 復習しましょう:

復習しよう – plain, casual, often used among friends/peers.
復習しましょう – polite volitional, used when being polite or formal.

In this sentence, the rest is in plain form (だった, 約束した), so 復習しよう matches that casual/plain level.

What is the role of と in 復習しようと約束した? Why is it this pattern?

Here と is the quotation particle. It marks the content of what was said, thought, or promised.

「復習しよう」 – “Let’s review.”
「復習しよう」と約束した。 – “(We) promised, saying ‘Let’s review.’”

So the structure is:

[volitional phrase] + と + 約束した
“promised to [do ~] / promised ‘Let’s do ~.’”

Other common patterns:

〜すると約束した – promised that (someone) will do ~
〜する約束をした – made a promise to do ~

Using the volitional + と約束した pattern emphasizes the shared intention: “We agreed/decided together: ‘Let’s do X.’”

Who is actually making the promise in みんなで一緒に復習しようと約束した? Where is the subject?

The subject is omitted, which is completely normal in Japanese. Context decides who it is.

Most naturally here:

(私たちは) みんなで一緒に復習しようと約束した。
– “We promised to review together, all of us.”

Even though 同級生の一人は is the topic of the first clause, the subject of 約束した is understood from みんなで:

Because one classmate failed, we all promised to review together.

So the implied meaning is “We (the group including the speaker) promised (each other).” The person or people to whom the promise was made are also omitted, because they’re obvious from context.

What exactly does 復習 mean? How is it different from 勉強 or 予習?

These three are related but not the same:

復習（ふくしゅう） – review / revision
– Studying something you have already learned to reinforce it.
– Ex: reviewing today’s lesson or past exam material.
勉強（べんきょう） – study in general
– Very broad; can be any kind of studying, new or old material.
予習（よしゅう） – preparation / preview
– Studying material you haven’t learned in class yet to be ready.

So in this sentence, 復習しよう means “Let’s review (what we’ve learned)” – it implies going back over existing material to help the classmate (and everyone) do better next time.