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.
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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.
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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.
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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.
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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’.
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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.
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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.