Question

Our friendship _ quickly over the weeks that followed。 为什么不能用was developing ？

Answer 1

the weeks that followed.是将来时态，that followed是weeks的定语，故不能用was developing。

Answer 2

develop不是瞬间动词，一般不用进行时

Answer 3

over the weeks that followed 记住这种结构，in the days that followed 全用一般过去时。在接下来的多长多长时间里。