Learn Hardcore Japanese: kare ha sudeni hensin wo okutta ga, tenpu ga nai. - He already sent a reply, but there’s no attachment.

は marks the topic (“as for him”). Using 彼が would mark him as the grammatical subject and can sound contrastive or focusy (“he (as opposed to others) sent it”). In neutral narrative, 彼は is more natural; use 彼が only when you want to emphasize who did it or answer “who?”

Do I need to say 彼 at all?

Often no. Japanese drops obvious subjects. If context is clear, you can say すでに返信を送ったが、添付がない。 In business, you’d usually use a name or role (e.g., 田中様は…) rather than 彼.

Why is が used between the two clauses? How is it different from けど/けれども/でも/しかし?

Clause-linking が means “but/however” and is slightly formal/written. Rough guide:

が: written-neutral to formal.
けど: casual; sentence-final softener in speech.
けれども/ですけれども: more polite/soft.
でも/しかし: start a new sentence; しかし is formal, でも is casual. Any could work; choose based on register.

Why is there another が after 添付?

This is the subject marker in the existential pattern Xがある/ない (“there is/there isn’t X”). 添付がない = “there’s no attachment.” If you say 添付はない, you topicalize “attachment,” adding a contrastive feel (“As for the attachment, there isn’t one [but something else might be true]”).

Is 返信を送った natural? How does it differ from 返信した or 送信した?

All are acceptable but nuanced:

返信した: “replied.” Common and concise for replying to a message.
返信を送った: literally “sent a reply.” Also fine; a bit more literal.
送信した: “sent (an email/message).” Not necessarily a reply; use for sending in general. In casual contexts, 返信した is the go-to. In UI/business, you’ll also see 返信済み (“reply sent/already replied”).

Why is を after 返信?

送る is a transitive verb that takes a direct object with を. So 返信を送る = “send a reply.”

What’s the difference between すでに, もう, and 既に?

すでに and 既に are the same word; 既に is the kanji form (more formal/printed). すでに in kana is very common.
もう also means “already,” but is more conversational and can carry a sense of speaker involvement/expectation.
All three fit here; すでに/既に feel a bit more formal or neutral in writing.

Why is it past in the first clause (送った) but present negative (ない) in the second? Should it be なかった?

Both are possible, with a nuance difference:

…送ったが、添付がない。 Describes a current state of that message: it (still) has no attachment.
…送ったが、添付がなかった。 Narrates the past situation at the time of sending. In many real-life contexts, the present ない feels natural because the email, as it exists now, lacks an attachment.

How would I make this more polite for an email?

Use です/ます forms and soften the second clause:

すでにご返信をいただいておりますが、添付ファイルが確認できません。
ご返信はいただいておりますが、添付ファイルが付いていないようです。
Even softer: 添付ファイルが見当たりませんでした。ご確認いただけますと幸いです。

Why can 添付 stand alone? Should I say 添付ファイル instead?

In email contexts, 添付 is often used as shorthand for “attachment(s).” 添付ファイル is more explicit and very common. Other options:

添付資料 (attached materials)
添付書類 (attached documents) All are fine; choose the most precise for the context.

What’s the nuance difference between 添付がない and 添付されていない?

添付がない: “There’s no attachment.” Short, neutral.
（ファイルが）添付されていない: “(The file) is not attached.” More descriptive, slightly more formal/technical.
Both are common; in polite emails, many prefer 添付ファイルが確認できません／付いていないようです to soften tone.

Where can すでに be placed?

Natural positions:

彼はすでに返信を送った…
彼は返信をすでに送った… (also OK)
すでに彼は返信を送った… (possible, slightly marked/emphatic) Avoid placing it after the verb: 送ったすでに is unnatural.

How do I indicate the recipient (e.g., “to me”)?

Add に:

彼は私に返信を送った。
彼は私に返信した。 For non-reply sending, you can also use A宛てに（メールを）送る (“send (an email) addressed to A”).

Could I replace the middle が with けど or ですが?

Yes:

Casual: …送ったけど、添付がない。
Polite: …送りましたが、添付がありません。
Very casual softener (sentence-final): …送ったんだけど、添付がない。

What are good ways to say “he forgot the attachment”?

添付を忘れた。
添付し忘れた。
添付するのを忘れた。 Polite/business:
添付を失念しておりました。
添付ファイルが抜けておりました。

Is mixing plain (ない) and polite (ました) forms okay?

Keep one register per sentence. The original is plain (送った／ない). In polite style, use 送りました／ありません（でした） consistently.

How do you read each word?

彼: かれ
すでに: すでに
返信: へんしん
送った: おくった
添付: てんぷ
ない: ない

Why is there a 、 and spaces between words?

The 、 separates clauses, like a comma. Standard Japanese does not use spaces between words; they were added here for learners. Normally you’d write: 彼はすでに返信を送ったが、添付がない。

What’s the difference among 返信, 返事, 回答, and 返答?

返信: a reply to a message (especially email/text).
返事: a reply/response in general (everyday).
回答: an answer to a question/survey/problem (more formal/technical).
返答: a reply/answer; formal/literary tone.