考试作文自认为写的没错,但改卷老师认为错了.

考试作文自认为写的没错,但改卷老师认为错了.
一：the river nearby was POLLUTED BADLY,I haven't even swum in the river.这句子你觉得哪里错了?注意大写处有无语序错误.
二：the water has become much clearer than before,and so,many people come to the river to have some fun.这句有几个地方.一是河水不可以用clear修饰吗?居然给我打错.而是 and so 那个地方,我在那里写了个逗号,and无实意,也就是说水变清澈了,因此,很多人去玩.so那里不能写逗号隔开吗?我主要是怕如果不用逗号的话句子会产生歧义,也就是会理解成so many people（很多人）而我要表达的是“因此,很多人……”.还有就是have some fun.请问可以用some修饰一下fun吗?
大家可以发表下自己的见解,