become为什么要变成became构成系动词

become(变的，变成)可以用来做系动词，它也是可以有不同的时态的。became是它的过去式，并不是说做系动词必须变成became. 而是两种形式都有可能，become用于一般现在时的句子，而became 用于一般过去时的句子。比如，They became friends./ He aims to become a teacher.