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.