Question

Now the air is much worse than it used to be. 谁能解释一下it 之前为什么不要加what.谢谢！。

Answer 1

我认为 it 之前是应该加（或可以加）what 的，只不过很多时候都可以把它省略掉而已。换言之，在 it 之前加 what 或不加 what 都可以。

以下是从外国网页复制下来的两个例子：
例子一：
1a. "You need to keep on using it and if you don’t, your skin will start to get even worse than what it was before." ［这句有 "what"］
1b. "Your skin will get even more worse than it was before if you stop using (product xxx)." ［这句没有 "waht"］
（以上两句均来自：http://womenlovebeauty.com/product-review/anti-aging-moisturizer/6840）

例子二：
The cumulative Current Budget Deficit in March (€3,706 million) is over 40% worse than what it was at this time last year (€2,613 million). And for March alone the Current Budget Deficit is over 50% worse than it was in the same month last year. ［第一句有 "what"；第二句没有 "what"］］
（以上句子均来自：http://economic-incentives.blogspot.com/2010/04/exchequer-balance-stops-getting-worse.html）

Answer 2

Now the air is much worse than it used to be这里的it指代的是前面的the air,因为在同一句话中重复了，所以用it指代，it已经充当主语成分了，根本不缺主语，所以不需要用what

追问

谢谢，但是比如 twice the size of what it used to be .中我们认为what 作表语,那than it used to be 中不能说也缺个表语呢.迷惑中....期待你的再次回答,再次谢谢.