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asita ha otouto no tanzyoubi nanode, oiwai ni keeki wo tukurimasu.

明日あした　は　弟おとうと　の　誕生日たんじょうび　なので、　お祝いわい　に　ケーキ　を　作つくります。asita ha otouto no tanzyoubi nanode, oiwai ni keeki wo tukurimasu.
Tomorrow is my younger brother’s birthday, so I’ll bake a cake to celebrate.
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Questions & Answers about asita ha otouto no tanzyoubi nanode, oiwai ni keeki wo tukurimasu.

What does は after 明日 signify? Why not が?

The particle は marks 明日 (“tomorrow”) as the topic of the sentence—the thing we’re talking about—rather than as the grammatical subject. In Japanese, time expressions are often set as topics to give context. If you used が, you’d be simply marking “tomorrow” as the subject, which sounds less natural for stating a time context.

How does 弟の誕生日 work? What is the role of の here?

The particle の shows possession or a close relationship between two nouns. Here, 弟の誕生日 literally means “younger brother’s birthday.” So, 弟 (“younger brother”) modifies 誕生日 (“birthday”).

Why is it 誕生日なので instead of just attaching ので to 誕生日 or saying だので?

When you attach ので (“because”) to a noun or a na-adjective, you need to insert な before ので. Grammatically:

  • verb/い-adjective + ので (no extra な)
  • noun/na-adjective + な
    • ので
      That’s why 誕生日 (a noun) becomes 誕生日 + な + ので → 誕生日なので.
What’s the difference between ので and から for giving reasons?

Both mean “because,” but:

  • ので is softer and more explanatory; it emphasizes an objective reason and often appears in writing or polite speech.
  • から is more direct and common in casual speech.
    You could say 明日は弟の誕生日だから…, but ので feels slightly more formal and polite here.
Why is there に after お祝い (as in お祝いにケーキを作ります)?

The particle に indicates purpose. お祝いにケーキを作ります means “I’ll make a cake for (the purpose of) celebrating.” In English: “I’ll make a cake for the celebration.”

Who is making the cake? The sentence doesn’t show a subject.

In Japanese, subjects are often omitted if they’re clear from context. Here, the implied subject is “I” (私). So the full idea is 私はケーキを作ります, but since you’re talking about your own plan, you can drop 私は.

Why is ケーキ written in katakana here?

ケーキ is a loanword (from English “cake”), and all loanwords in Japanese are written in katakana. Native Japanese words use hiragana or kanji.

Why use the polite form 作ります instead of the plain 作る?
The speaker chooses a polite register, probably because they’re talking to someone they’re not extremely close with (e.g., at work or in a formal setting). 作ります is the masu-form (polite present/future), while 作る is the plain/dictionary form used in casual speech or writing.