have been 与had been

have been变成had been的情况一般出现在宾语从句中.因为宾语从句的时态要与主句谓语动词时态一致.比如：
He has been in China for 4 years.这是has been.但变成宾语从句的时候,如果主句是过去式,这里也要变成had been.例子：
Tom said he had been in China for 4 years.汤姆说他在中国呆了4年了.由于主句的谓语said是过去式,所以has been变成了had been.