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siken no tame ni hon ga hituyou desu.

試験しけん　の　ため　に　本ほん　が　必要ひつよう　です。siken no tame ni hon ga hituyou desu.
Books are necessary for the exam.
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Questions & Answers about siken no tame ni hon ga hituyou desu.

What does the particle の do in 試験のために?
The の here is the genitive (possessive) particle linking 試験 (exam) to ため (sake/purpose). Together, 試験のため literally means “for the sake of the exam” or more naturally “for the exam.”
Why is it ために and not just ため?

ため by itself is a noun meaning “sake,” “benefit,” or “cause.” To turn it into a phrase that expresses purpose (“in order to…”) or reason (“because of…”), you add the particle に.

  • Noun + の
    • ために = “for the purpose of…” (purpose)
  • Noun + の
    • ため (often in writing) = “because of…” (cause/reason)
      In this sentence, 試験のために is used for purpose: “in order to prepare for the exam.”
Why is 本 followed by が instead of を?
The phrase 本が必要です treats 本 (book) as the subject of the adjective 必要 (necessary). In Japanese, when you say Xが必要です, you are literally saying “X is necessary.” Thus が marks the thing that is needed, not a direct object.
What kind of word is 必要 and how does it work with です?

必要 is a na-adjective (some call it a noun in adjective disguise) meaning “necessary.”

  • With です: 必要です (polite)
  • Plain form: 必要だ (casual)
    You always mark the needed item with が: Xが必要です = “X is necessary.”
Why isn’t there a subject like “I” in the sentence? Can I add one?

Japanese often omits an obvious or context-known subject. Here, I am implied as the one who needs the book. If you want to be explicit, just add 私は at the front:
私は試験のために本が必要です。

Could I use は instead of が in 本が必要です? What changes?
Yes, you can say 本は必要です, but now 本 becomes the topic (“As for the book, it is necessary”), which can imply contrast or emphasis on 本 versus something else. 本が必要です is a neutral statement: “A book is needed.”
How do I say “I need two books for the exam”?

Insert the counter 二冊 (にさつ) before 必要:
試験のために本が二冊必要です。
Literally: “For the exam, two books are necessary.”

Can I use the plain form 必要だ instead of 必要です? Is there any nuance?

Yes.

  • 必要です – Polite/formal
  • 必要だ – Plain/informal (often more masculine)
    Choose です in polite conversation or writing, and だ among friends or in casual contexts.