He bought many books yesterday.(他昨天买了很多书).bought和booksbought和books的形态变化所表示的语法意义和语法范畴.

（4）This is the pen（which）he bought yesterday. 这是他昨天刚买的（2）I bought many books yesterday, three of which are written by Lu

bought 在这里根据yesterday，确定用一般过去式，故用buy的过去式bought,
books 根据前面的many， many+可数名词复数，所以用books
望采纳`~

bought 是buy的过去式,这里受时间的限制用过去时.这个动词做谓语.books是复数在这里做宾语.

不明白你的问题，不过试着回答。
bought 是bring 的过去式，是时态变化
books是book 的复数形式，是可数名词的单复数变化。