定语从句that which一起用的情况

as ／which引导非限制性定语从句。 

引导词as和which的区别在于： 
①as引导的非限制性定语从句可放于主句前、主句中、主句后，而which引导的非限制性定语从句一般放于主句后或句中。as放在句首的情况常用的句式 有as we all know,as is mentioned above,而which我们则看不到这样用于句首的情况。
②as常与从句中的know，see， hear，expect等动词连用，也常用于as often happens，as is often the case（常有的事）等句子中；而which一般不用于以上情况。 
③as有“正如”的含义，which没有此含义。如： 
as is well known，shanghai is a big city． 
some of the roads were flooded， which made our journey more difficult．