了 is usually used after a verb or adjective, means someone has already done sth, action finished or already in a state.
我看了天气预报：I have watched the weather forcast.
be hugry/饿了：I‘m hungry/我饿了
have eaten/吃了:I’ve eaten/我吃了
我睡了:I’m already laid down and about to sleep.
长大了：have/has grown up.
be crazy/疯了：你疯了吗？
fruits are ripen/果实熟了：树上的苹果熟了。
红了，绿了，黄了…: getting/turn red, getting/turn green, getting/turn yellow, …
更漂亮了：become more beautiful