～になる / ～くなる【になる / くなる】

なる means "become". A noun or a な-adjective takes に in front of it, and an い-adjective changes its final い to く: いしゃ医者になる, きれいになる, たかく高くなる.

The thing that changes takes が or は.

に or く

The choice follows the type of word, not the meaning. An い-adjective never takes に, and a な-adjective never takes く. きれい and ゆうめい有名 end in い but are な-adjectives, so they take に.

• × 高いになった → 高くなった
• × きれくなった → きれいになった

いい changes from its other form, よい: よくなる, never ×いくなる.

なる or する

なる says that the change happened. する says that someone made it happen, and the thing changed takes を.

Usage notes

These go beyond becoming something new.

Stopping: 〜なくなる

A negative ending in ない changes like an い-adjective: ない → なく. On a verb it means you stopped doing something; on a noun or an adjective, it stopped being so.

Ages and seasons

An age or a season takes になる where English says "turn" or "come".