在一个英语句子中如果前面有一个词为过去式 那么后面应该也为过去式吗

那有多种情况：
如果后面是宾语从句,则通常是与过去有关的时态
He said that he was at home yesterday.(was一般过去时)
He said that he had been to Beijing twice.(had been过去完成时)
He said that he would go home tomorrow.(would go过去将来时)
He said that he was watching TV at eight yesterday evening.(was watching过去进行时)
He said that the earth goes around the sun.(goes一般现在时)
只有在所说的话是真理的情况下用一般现在时.
如果后面不是从句,那就通常用动词不定式、动名词、分词等形式.也就是非谓语形式.（较复杂,最好看语法书）
如
He went to buy a pen.(to buy)
He enjoyed playing computer games yesterday afternoon.(playing)
He saw the boy come out from the room.(come)
太多了,打起来麻烦,这要上语法课都要上很多节的,一下二下难说清楚的.