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