need; necessary
Expresses that something is needed or necessary.
This pattern states that a thing or action is required. It is a straightforward way to say "need" in Japanese. The noun that is needed is marked by が, and 必要 is a na-adjective/noun. You often see 必要です in polite speech and 必要だ in casual speech. For example, お金が必要です means "Money is necessary." To express needing to do a verb, you can nominalize the verb with こと, as in 予約することが必要です (It is necessary to make a reservation). This is a bit formal; in conversation, people often use なければなりません or たほうがいい instead. But が必要 is perfectly fine and common in both written and spoken Japanese when stating a requirement.