1.I can think of many cases___students obviously knew a lot

1.I can think of many cases___students obviously knew a lot of English words and expressions but couldn't write a good essay.
为什么用where 不用which
2.In order to study English well,___
my father boughe me a dictionary有错么,为什么用i bought a dictionary
3.Although he is considered a great writer,___
在his works are not widely used 前+still为什么不对
4.If you want to be a writer ,I suggest that attention should be paid____the works of LunXun.
不是应该是be paid to de 为什么是to reading.
5.The classroom is so dirty that it needs ____badly.
用cleaning,这个need的用法搞得不是很清楚,