Grammar JLPT Level N4 - Lesson 22

が必要(ひつよう) is a common expression in Japanese which is used to highlight the 'need' or 'necessity' of something. It is a structure that may only be used with nouns, which means that verbs will need to be nominalized (turned into noun-phrases) in order to use this construction.

• プールで泳(およ)ぐときは水着(みずぎ)が必要(ひつよう)だ。
  When swimming in a pool, a swimsuit is necessary.
• バスを運転(うんてん)するには特別(とくべつ)な免許(めんきょ)が必要(ひつよう)だ。
  You need a special license to drive a bus.
• 彼(かれ)は車(くるま)を持(も)っていないから迎(むか)えに行(い)くことが必要(ひつよう)だ。
  Because he doesn't have a car, it is necessary to go pick him up.
• ここは泥棒(どろぼう)が多(おお)いから出(で)かける前(まえ)に鍵(かぎ)をかけることが必要(ひつよう)だ。
  Because there are a lot of burglars here, it is necessary to lock your door before you go out.

Both が and だ may sometimes be omitted from this expression in casual speech. However, it is a good idea to at least use が, to clarify the thing that is 'necessary'.

• 私(わたし)は運転(うんてん)するときは眼鏡(めがね)が必要(ひつよう)。
  I need glasses when I drive.
• 田舎(いなか)に住(す) むには車(くるま)が必要(ひつよう)。
  To live in the countryside, a car is necessary.