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