1)非限制性定语从句要用which, who, whose等词引起，它们可以修饰主句中的某个名词，也可修饰这个句子，起一种附加说明的作用，但不能用that引导。如把它省去，主句仍能表达明确完整的概念。
,all of ＋ which/ whose/ whom
,most of …
,many of …
, … of
l We toured a 300-year-old house.
l The exterior of the house consisted of logs cemented with clay.
We toured a 300-year-old house, the exterior of which consisted of logs cemented with clay.
l They own an original Picasso painting.
l The value of the painting is more than a million dollars.
They own an original Picasso painting, the value of which is more than a million dollars.
As is shown in…
As has been stated above…
As has just been proved…
As is well known…
As we know…
As has been pointed out…
He is always late, generally managed to annoy everybody.
A) which B) where
C) what D) that
is known to the world, Mark Twain is a great American writer.
A) That B) Which
C) As D) It