改一句句子

The correct expressions are:
I took part in many competitions when I was a student.
or
I participated in many competitions when I was a student.

Joined is not the correct word for your sentence. You join a club meaning you become a member of the club but you do not join a competition.

We can correctly say "join a competition", "join many competitions", "join Boston Marathon", "join the Olympic Games", etc.

沒有錯。讀起來是非常通的。

沒有錯
但我見你應該想表達:我是一個學生時,參加過多項比賽
我認為 I participated many competitions when I was a student. 較適合
因為join有加入的意思,多用於我加入甚麼
E.g. I will join English drama club next year.
而paticipate就比較正確
不過這樣亦沒有錯
(只是我的一孔之見)

我唔知你本身想講乜,但係就咁睇文法就冇錯:)