When something ISN’T AS GOOD (PRETTY, FAST..) AS something else, we use 没有 /méiyǒu/ instead of 比 /bǐ/
Her hair isn't as long as before.
她的 头发 没有 以前 长。
Chinese isn't as difficult as Japanese.
汉语 没有 日语 难。
Their relationship isn't as good as before.
他们 的 关系 没有 以前 好。
Chinese food isn’t as good as Japanese food.
中国菜 没有 日本菜 好吃。
My Chinese isn't as good as his.
我的 中文 没有 他 (的) 好。
Beijing isn't as safe as you think.
北京 没有 你 想 得 那么 安全。
America isn't as big as China.
美国 没有 中国 大。
This isn’t as good as that.
这个 没有 那个 好。