那有多種情況：
如果后面是賓語從句,則通常是與過去有關(guān)的時(shí)態(tài)
He said that he was at home yesterday.(was一般過去時(shí))
He said that he had been to Beijing twice.(had been過去完成時(shí))
He said that he would go home tomorrow.(would go過去將來時(shí))
He said that he was watching TV at eight yesterday evening.(was watching過去進(jìn)行時(shí))
He said that the earth goes around the sun.(goes一般現(xiàn)在時(shí))
只有在所說的話是真理的情況下用一般現(xiàn)在時(shí).
如果后面不是從句,那就通常用動(dòng)詞不定式、動(dòng)名詞、分詞等形式.也就是非謂語形式.（較復(fù)雜,最好看語法書）
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,打起來麻煩,這要上語法課都要上很多節(jié)的,一下二下難說清楚的.