我把剑桥语法的例句和解释给你,你自己去理解吧。
In this section we are concerned with term comparisons of equality where the preposition as appears on its own, without a superordinate comparative governor such as same, such, so, or a preceding as:
[27] i As you know, we face a difficult year. [adjunct of comparison]
This as is itself the comparative governor, and takes a bare comparative complement (cf.§1.3).
[28]i [As I have already observed ___ ,] no reason has yet been offered for this change.
ii The event was sponsored, [as ___ is the fashion these days,] by a brewery.
The comparative clauses functioning as complement to as are structurally incomplete in that the clausal complements which their verbs would have in main clauses are missing. They are recoverable from the matrix: in [28i], for example, what I have already observed is that no reason has yet been offered for this change. Similarly in [ii]: what is the fashion these days is for comparable events to be sponsored by a brewery.