只要看which or that 所指代的先行词在句子中做什么成分.
A prosperity which / that had never been seen before appears in the countryside.（which / that）指代的内容是A prosperity在定语从句A prosperity which / that had never been seen before中作主语.即
A prosperity had never been seen before.
a car （that） can travel 150 miles an hour.（作主语）
The car that he drives can travel 150 miles an hour.
{The car can travel 150 miles an hour.这是主句部分.}
拿着the car 往后面定语从句部分（he drives）去放,应放在 he drives 后..也就是做drive 的宾语.