Learn Hardcore Japanese: ninki no resutoran ha yoyaku ga hituyou desu. - Popular restaurants require a reservation.

It links one noun to another, making an attributive phrase. Here 人気 (popularity) + の modifies レストラン (restaurant), giving “restaurant(s) with popularity,” i.e., “popular restaurant(s).” の often works like “of” or turns a noun into a descriptor: 日本の映画 (Japanese film), 子どもの本 (children’s book).

Can I say 人気なレストラン instead of 人気のレストラン?

You may hear 人気な in the wild, but the most standard and safe choice is 人気のレストラン. Another very natural option is 人気のあるレストラン (“a restaurant that has popularity”). If you’re not sure, stick with 人気の or 人気のある.

What does は after レストラン mean here?

は marks the topic: “As for popular restaurants, ….” It sets the scene and often implies a general statement. If you want to emphasize “at popular restaurants (as a location),” use では: 人気のレストランでは予約が必要です.

Why is 予約 marked with が and not を?

Because this uses the pattern Nが必要です (“N is necessary”). 予約 is the thing that is necessary, so it takes が. You’d use を when 予約 is the direct object of a verb like する: 予約をする (“to make a reservation”).

Can I say 予約は必要です instead of 予約が必要です?

Yes, but 予約は必要です tends to add contrast or topicalization (“As for reservations, they’re necessary”), implying maybe other things aren’t. The neutral, default statement is 予約が必要です. Example: 予約は必要ですが、会員登録は不要です (“Reservations are required, but membership registration isn’t.”)

Is 必要 a noun or an adjective? How does it work?

必要 is a noun/na-adjective. As a predicate: 必要です／必要だ (“is necessary”). To modify a noun, use 必要な: 必要な書類 (“necessary documents”), 必要な情報 (“necessary information”). In this sentence it’s used predicatively: 予約が必要です.

How else can I say “A reservation is necessary”?

予約する必要がある／あります: “It’s necessary to make a reservation.” (focus on the action)
予約が要ります（いります）: “A reservation is needed.” (more everyday)
予約は不要です／いりません: “No reservation needed.” (negatives)
Very formal/notice style: 予約が必要となります or 予約が必要です。

Why does the sentence end with です and not だ?

です is the polite form; だ is plain/casual. Polite: 人気のレストランは予約が必要です。 Casual: 人気のレストランは予約が必要だ。 Negative polite: 必要ではありません／必要じゃありません; casual: 必要じゃない.

Can I drop the topic and just say 予約が必要です?

Yes. If the context (e.g., a specific restaurant) is clear, 予約が必要です alone is perfectly natural—common on signs and websites.

Do I need で after レストラン (レストランでは)?

Use では when you want to highlight the place: 人気のレストランでは予約が必要です (“At popular restaurants, a reservation is necessary”). Without で, 人気のレストランは… is a broader topical statement about that category.

How do I ask “Do I need a reservation?” naturally?

General/polite: 予約は必要ですか。
Softer/polite: 予約は必要でしょうか。
Everyday: 予約はいりますか。 For a specific place, use the name or このレストランは予約が必要ですか。

How do I read the kanji in this sentence?

人気: にんき
レストラン: resutoran (katakana loanword)
予約: よやく
必要: ひつよう

What’s the word order logic here?

Modifiers come before the noun in Japanese: 人気の (popular) + レストラン (restaurant). Then the topic は, and the comment 予約が必要です (“a reservation is necessary”). So it’s literally: “As for popular restaurants, a reservation is necessary.”