To express that something 'becomes (A)' or undergoes a change, Japanese uses になる (for な-Adjectives and nouns) or くなる (for い-Adjectives). Because this grammar uses the verb なる, it can be conjugated to show that a change will occur (non-past), is in a resulting state (continuous), has already occurred (past), or will not occur (negative).

• もうすぐ空(そら)が暗(くら)くなるよ。
  The sky will get dark soon.
• かれの顔(かお)が赤(あか)くなっている。
  His face has turned red.
• テレビを見(み)ていたら、眠(ねむ)くなった。
  I got sleepy while watching TV.
• 練習(れんしゅう)しないと、上手(じょうず)にならないよ。
  If you don't practice, you won't get good at it.

Notice in the example of the face turning red, the ている form describes a resulting condition. This implies the action of 'becoming red' is complete, and the face is now in a continuous state of 'being red'.

The adjective いい (good) is an important exception. When indicating that something 'becomes good', it changes to よくなる instead of the expected いくなる.

• ちゃんと薬(くすり)を飲(の)めば、病気(びょうき)は良(よ)くなるでしょう。
  If you take your medicine properly, your illness will probably get better.