Learn Hardcore Japanese: yuugata made ni syusseki ka kesseki ka wo apuri de okutte kudasai. - By evening, please send whether you will attend or be absent via the app.

までに = “by (no later than).” It sets a deadline at a point in time.
まで = “until (up to and including).” It implies something continues up to that point. So 夕方までに送ってください means “Send it by evening.” 夕方まで送ってください would sound like “Keep sending until evening,” which isn’t the intent here.

Why are there two か in 出席か欠席か?

When an A-or-B choice is followed by another particle (here, を), you close the choice with a second か so the whole unit “出席 or 欠席” is treated as one chunk: 出席か欠席かを. Without the second か, を would look like it applies only to 欠席. At the end of a sentence you often see just AかB, but inside a larger sentence “AかBか” is common.

What is を marking here?

It marks the direct object of 送ってください. The thing being sent is the content “出席か欠席か” (“attendance or absence”).

Why アプリで and not アプリに or アプリから?

で = by means of/via: アプリで送る (“send via the app”).
に = recipient/destination: 先生にアプリで送ってください (“send it to the teacher via the app”).
から = source: アプリから送る emphasizes “from within the app.” Here “via the app” is intended, so アプリで is natural.

Is 送ってください the right politeness level? Any alternatives?

送ってください is standard polite. Other options:

More technical: 送信してください / ご送信ください
More formal/polite: 送っていただけますか, ご送信いただけますと幸いです
Different nuance: 連絡してください (“please let me know”), 知らせてください (“please inform me”), 提出してください (“please submit”) depending on context.

Difference between 送る, 送信する, 連絡する, 提出する?

送る: general “send” (things/messages).
送信する: “transmit/send” in a digital/technical sense (email, forms). Often a button label.
連絡する: “contact/notify” (focus on communication).
提出する: “submit” (assignments/forms). Choose based on purpose and formality.

Are 出席 and 欠席 nouns or verbs? What about 出欠?

出席/欠席 are nouns that can be verbal with する: 出席する/欠席する.
In this sentence they’re plain nouns listed as alternatives.
You can also say 出席するか欠席するかを… with the verbs.
出欠 (しゅっけつ) means “attendance status.” Example: 夕方までに出欠をアプリで送ってください.

Could I use かどうか instead of AかBか?

Yes: 出席するかどうかをアプリで送ってください = “Please send whether you’ll attend or not.” AかBか explicitly names both choices; かどうか frames it as “whether or not.” Both are fine. Another natural phrase is 出欠の連絡をアプリでしてください.

Where is “you” or the subject of the sentence?

Japanese often omits obvious subjects. With 〜てください requests, the understood subject is “you”: “(You) please send ….” The recipient (teacher/organizer) is also implicit from context.

Can I change the word order?

Yes, as long as the verb stays at the end and chunks stay intact. For example:

アプリで、夕方までに、出席か欠席かを送ってください。
夕方までに、アプリで、出席か欠席かを送ってください。 Don’t split 出席か欠席か.

Are the spaces between words normal Japanese?

No. They’re for learner clarity. Normal writing is: 夕方までに出席か欠席かをアプリで送ってください。

Why is ください written in hiragana, not 下さい?

As a request auxiliary (“please do X”), ください is usually in hiragana. 下さい (kanji) is mainly for the main verb meaning “give (me) [something],” e.g., 水を下さい (though hiragana is common there too). So 送ってください is standard.

How do I pronounce it? (reading and romaji)

Reading: ゆうがたまでにしゅっせきかけっせきかをアプリでおくってください。
Romaji: yuugata made ni shusseki ka kesseki ka o apuri de okutte kudasai.

What time range does 夕方 usually mean? Does it imply “this evening”?

夕方 typically means early evening (roughly 4–7 pm, depending on season/context). Without a date, it usually implies “this evening” (today). For other times, specify: 今日の夕方までに, 明日の夕方までに, or a clock time (e.g., 19時までに).

How do we get 送って from 送る?

送る is a godan (u-verb). Te-form rules: う/つ/る → って. So 送る → 送って. Add ください for a polite request: 送ってください.

How can I add the recipient explicitly?

Use に for the recipient: 先生に、夕方までに出席か欠席かをアプリで送ってください。 = “Please send (to the teacher) via the app by evening whether you’ll attend or be absent.”