求助一句话语法问题

At that time，没有问题
担心用worried about比较好，so that 后面缺少主语they
didn't buy milk,milk 同一句话第二次出现避免重复用，应用it

同意四楼的

at that time, people didn't buy milk because of worrying the quality of milk. 这样比较好

就时态来说，were和didn't是对的，因为陈述的是过去事实。
我认为 这句话有语法问题 ，因为你用的短语不正确 。

At that time , people were so concerned about the quality of milk that they didn't buy milk.
或者 At that time , people were concerned about the quality of milk so that they didn't buy milk.

be concerned zbout 是担心的意思，比你用的afraid of 要好 ， afraid of 是惧怕的意思

个人认为
At that time ,people were worried about quality of milk,so they didn't buy milk instead.
更地道

At that time , people were so worried about the quality of milk that they didn't buy it.
这里不是be afraid of 是因为句中为担心牛奶质量，而非害怕，其它改动楼主应该看得懂吧.