在说或写一句话时，通常要选择一个中心时态，这个中心时态影响其它时态 ，即时态要前后一致。
以现在为中心：若开始叙述或描述着眼于现在，则常以现在为起点，从而会出现一般现在时、现在进行时、现在完成时或一般将来时连用的情况。
Our postman usually delivers our arrival at 7 every morning. It (be) nearly lunchtime and the mail still (not arrive). I (suppose) the mail (come) soon. Perhaps he (be) ill. (is, hasnt arrived, suppose, will come, is
以过去为中心：若着眼于过去，则常以过去某时为基点，从而出现一般过去时、过去进行时、过去将来时和过去完成时连用的情况。
We lived in the street. They (build) houses all around us then. We (be) there for 10 years and (imagine) we (stay) there for the rest of our lives. (were building, had been, imagined, would stay)