Subject + 变 + 了
变 (biàn) means "to change" or "to become." It indicates a transformation or shift in state. When used without a complement it emphasizes that a change has occurred, and it often appears with 了 (le) to mark the completion of that change.
天气突然变冷了,你多穿点衣服。
Tiānqì tūrán biàn lěng le, nǐ duō chuān diǎn yīfu.
The weather suddenly turned cold; put on more clothes.
他出国几年,整个人都变了。
Tā chūguó jǐ nián, zhěnggè rén dōu biàn le.
He went abroad for a few years, and he's completely changed as a person.
树叶到了秋天就会变黄。
Shùyè dào le qiūtiān jiù huì biàn huáng.
Leaves turn yellow when autumn comes.