我相信他是我见过的最高的英语老师

I believe( that) he is the tallest English teacher I've ever met.
这是根据字面意义翻译，我觉得believe换成think会更好一些。

I believe he is the highest I have seen English teacher

不会

I believe that he is the tallest English teacher i have seen.

正解：I believe that he is the tallest english teacher I have ever seen

不对，是I believe that he is the tallist（此处不敢确定，可能拼写错误） English teacher I've ever seen