Learn Hardcore Japanese: kare ha tikokusita node, senpai ni ayamatta. - He was late, so he apologized to his senior.

は marks the topic: “as for him.” It frames what follows as information about him. If you use が in the reason clause—彼が遅刻したので—you highlight that it was he (as opposed to someone else) who was late. With 彼は, the topic “he” is established and both clauses are understood to be about him. Both are grammatical; choose は for topic-setting, が for subject emphasis.

Why use ので? How is it different from から? Are they interchangeable?

Both mean “because,” but:

ので is softer, more explanatory/objective, and often sounds more polite or formal.
から is more direct and conversational, often expressing the speaker’s personal reason or decision. You can say 彼は遅刻したから、先輩に謝った with a slightly stronger, more casual tone. In explanations or to superiors, ので is safer.

How does ので attach to words? Do I ever need な?

Verb/i-adjective: plain form + ので (e.g., 遅刻したので, 忙しいので)
Noun/na-adjective: add な
- ので (e.g., 学生なので, 静かなので)
Polite style is fine: しましたので, 忙しいので.

Can I put the clauses in the opposite order?

Not with …、遅刻したので at the end; that sounds off. Natural alternatives:

遅刻したので、先輩に謝った。 (reason first)
先輩に謝ったのは、遅刻したからだ。 (focus on the reason)
Two sentences: 先輩に謝った。遅刻したからだ。

Why is it 先輩に and not 先輩を? Could I use へ or と?

The verb 謝る takes the target with に: “apologize to someone.”

先輩に謝る = apologize to the senpai.
へ is directional and not used here.
と marks quoted content: 先輩に「すみません」と謝った (said “sorry” to the senpai).

What exactly does 先輩 mean? Is it “boss”? How do you address them?

先輩 (senpai) is a senior member in the same school, company, club, etc. It’s not necessarily your formal boss (that’s often 上司). You might address someone as 田中先輩, or more commonly use their name + さん. Use 先輩 as a relationship term, not as a universal honorific for any older person.

What’s the difference between 謝る and phrases like すみません, ごめんなさい, 申し訳ありません?

謝る = “to apologize” (the action/verb).
すみません/すみませんでした = polite “sorry/excuse me” (very common).
ごめんなさい/ごめん = casual “sorry.”
申し訳ありません/ございません = very formal/humble apology. You can combine them: 先輩に「すみません」と謝った.

Is 遅刻する the same as 遅れる?

遅刻する: a person is late for a scheduled obligation (work, class, meeting). Ex: 会社に遅刻する, 授業に遅刻した.
遅れる: “be late/delayed” more generally (people or things). Ex: 電車が遅れる; also usable for people: 会議に遅れた.
Nuance: 遅刻 is specifically “tardiness” to an appointment; 遅れる is broader.

How do I say what he was late for? Does 遅刻 take an object?

Use に to mark the event/place:

会議に遅刻した, 学校に遅刻した, 会社に遅刻した. With 遅れる, you also use に: 授業に遅れた.

Do I need to say 彼? Can I drop the pronoun?

Japanese often omits obvious subjects. If context makes it clear, you can say simply:

遅刻したので、先輩に謝った。 In polite speech: 遅刻したので、先輩に謝りました。

How would I make the sentence more polite or formal?

Polite: 彼は遅刻したので、先輩に謝りました。
More formal/businesslike: 遅刻しましたので、先輩にお詫びいたしました。 or …謝罪しました。
Very humble set phrase when apologizing: 大変申し訳ありませんでした。

Is the comma after ので necessary?

It’s optional, used for readability. Japanese often inserts a comma after a reason clause: …ので、…. Without it is also correct.

Are there other connectors I could use instead of ので, like ため or せい? What about the て-form?

〜ため（に） (reason): formal/neutral. 遅刻したため、先輩に謝った。
〜せいで: negative cause with a blaming nuance. 遅刻したせいで、先輩に怒られた。 (Sounds like “because of (someone’s) fault.”)
Vて: sequence/result, often implying cause in casual style. 遅刻して、先輩に謝った。 (“[I] was late and (so) apologized.”)
Casual contraction: 遅刻したんで、先輩に謝った。 (んで ≈ ので, but more colloquial.)

How do you pronounce the whole sentence?

Kare wa chikoku shita no de, senpai ni ayamatta.
In kana: かれはちこくしたので、せんぱいにあやまった.