It is widely believed that the pull of gravity on a falling raindrop changes the drop' round shape into a teardrop shape.句中的it 是形式主语,正式主语是that引导的从句,称为主语从句,由于正式主语太长了,所以,用it代替,并且后置.
在主语从句里,the pull of gravity on a falling raindrop 是主语从句的主语,changes是主语从句的谓语,the drop' round shape into a teardrop shape是主语从句的宾语.