When something ISN’T AS GOOD (PRETTY, FAST..) AS something else, we use 没有 /méiyǒu/ instead of 比 /bǐ/
EXAMPLES
Beijing isn't as safe as you think.
This isn’t as good as that.
这个 没有 那个 好。
Zhège méiyǒu nàge hǎo.
Her hair isn't as long as before.
Chinese isn't as difficult as Japanese.
America isn't as big as China.
美国 没有 中国 大。
<Měiguó méiyǒu Zhōngguó dà.>
My Chinese isn't as fluent as his.
My Chinese isn't as good as his.
我的 中文 没有 他 (的) 好。
<Wǒde Zhōngwén méiyǒu tā hǎo.>
Their relationship isn't as good as before.