くなる details
Examples
becomes; gets — a change of state in an i-adjective quality
最近、日が暮れるのが早くなったね。
saikin, hi ga kureru no ga hayaku natta ne.
The sun's been setting earlier lately, hasn't it? casual
薬を飲んだら、だいぶ良くなりました。
kusuri o nondara, daibu yoku narimashita.
After taking the medicine, I got quite a bit better. neutral
荷物が多いと、動きが遅くなります。
nimotsu ga ooi to, ugoki ga osoku narimasu.
With a lot of luggage, your movements get slower. neutral
Formation
i-adj stem + くなる（高くなる）attaches to i-adjective
Notes
いい changes on its よ stem: 良[よ]くなる, never ×いくなる. Standalone
なくなる (from ない) means "run out; disappear" — 時間[じかん]がなくなった.
Related patterns
Similar
The same "become" split by word class — nouns and na-adjectives take になる (医者になる、静かになる), i-adjectives take stem+くなる (高くなる); the two never swap hosts, so the choice is grammar, not nuance.