✔ 最佳答案
Simple sentence 一個句子只有一個 main verb. 但是 compound sentence, 可以有多一個動語, 不限於 past tense。
你的例子屬 compound sentence:
I said (that) I was going to be a teacher which is the best in the world.
that I was going to be a teacher 是一個子句。子句有自己的 subject and verb.
which (a teacher) is the best in the world 也是一個子句, 有自己的 subject and verb.
第二句子句,is 改用 would be 或 will be 會比較恰當。
Reported speech 中,當複述的是事實或真理,會使用 present tense. 但這裡只是一個夢想,因此would be 或 will be 會比較恰當。