Hello, glad to help u. “是“ is a verb just like "be" in English, it can be "is"/"am"/"are". It is usually used for statement. but, ”当“ is like "when" in English. I can show you some examples. 1.我“是”一名英语老师(I 'am' an English teacher.) 2.当我在美国旅游时，我去了很多地方。(I have been to many places 'when' I traveled in USA.) If you have any questions about it, just ask me :D

是means“yes”and“be”,当 means “when”and “if”

我是老师 and 我当老师 have the same meaning while, 是 means 'am' or 'is' , 当 means a state that you can easily change it. just like 'act as' for example, I am Tom.we can say 我是Tom,but not 我当Tom you can add occupation after 当,when we say I am a teacher. we can say 我是老师 or 我当老师. or you can use it to explain assumption, let's play a game,you are student and I am teacher. 我们玩游戏，你当学生，我当老师。 当 also means 'when'. for example, when I was seven.当我七岁的时候

是 is a be-verb, 是 sth means the reality and the truth, 我是老师 is a state of real condition. 当is a action verb, 我当老师is an answer to what do you do, is similar to 我做老师

As I understand, 是 to be, 当 to work as (or perhaps to act as).