Lizzy is______(nice)as,if not ____(nice)than,Mary.

Lizzy is as nice as, if not nicer than, Mary

如果Lizzy不比Mary更好，至少她们一样好。

这样分析
Lizzy is as nice as(if not nicer than) Mary.
以前的英语老师经常告诉我们，看到前后有逗号的，就用括号括起来不要管它。

as nice, nicer.
相当于
if (Lizzy is not) nicer than Mary,Lizzy is as nice as Mary.括号内可省略。
as nice as Mary和玛丽一样漂亮
句子意思是：Lizzy至少不会没Mary漂亮

is 后面一定少了个as 两个空分别该为 (as)nice和nicer 意思是：lizzy如果不比马利好的话，至少也和她一样好。

nice: nicer
如果说丽兹没有玛丽更好得话，她至少也和玛丽不相上下。