英语问题..1English is a useful language.It is often used as a se

英语问题..1
English is a useful language.It is often used as a second language.
划下划线的a为什么不能用the替换?在序数词前不是应该用the吗?
The students from the United States live in Fifth street .
在Fifth 前不是应该用the吗?这里为什么不能用?
Mary will be made _____ monitor of her class. A.the B./ 请详细说明为什么•
What is the most useful invention in 20th century?
The computer 这里的computer为什么不加S,可以的吗?