Why is ひつよう considered a な adjective?

Question

Im struggling to understand why Hitsuyou (ひつよう) is considered a な adjective.

From what I understand after reading about い adjectives we simply drop the last い to make it negative, for example: さむい　becomes 住むくない because we dropped the last い and added くない to make cold become "not cold".

This is pretty simple to understand as each word we want to make negative usually has an い on the end.

However, using this same rule to use じゃない for な doesnt make sense to me with ひつよう as there is no な in the word.

Ive been reading through this online dictionary but still haven't been able to grasp why.

Can you help me understand?

Answer 1

For the so-called "-na adjectives", the -na only appears when the adjective is used attributively -- when it used to modify a noun. When these adjectives are used predicatively -- at the end of a sentence -- there is no -na.

Examples:

• この車【くるま】は静【しず】かです。 -- This car is quiet.
  これは静【しず】かな車【くるま】です。 -- This is a quiet car.
• この本【ほん】は必要【ひつよう】です。 -- This book is required / necessary.
  これは必要【ひつよう】な本【ほん】です。 -- This is a required / necessary book.

Note that じゃない negation can be used after both -na adjectives and nouns, and can be used both attributively (to modify a noun) and predicatively (at the end of a sentence).

• この車【くるま】は静【しず】かじゃないです。 -- This car is not quiet.
  これは静【しず】かな車【くるま】じゃないです。 -- This is not a quiet car. (It could be a loud car, or a slightly rattly car, or not even a car at all.)
  これは静【しず】かじゃない車です。 -- This is a not-​quiet car. (A bit clunky in both the English and Japanese, but the meaning is clear.)

There's some more information over at Wikipedia that might be useful.

If you are still confused, please comment and I can expand this answer.