Aliza, if you are truly at A0 level, and even if you are a bit further on, you really should not consider looking at lessons so far beyond your current level. There is more than enough to come to grips with in earlier lessons.

In this case, these are 2 different past tenses - imparfait and passé simple. Translated to English, both can be represented by ‘ I was’ as you note. 

In speech it is imparfait and passé composé you need to know - and that is a huge learning curve already ! Passé simple is almost non-existent in modern spoken French, so ‘je fus..’ shouldn’t bother you at this stage. 

If you progress systematically, you will understand the context and perspective much better when you need to (if ever, for passé simple).