I have a problem with correct understanding difference while I should use past simple continuous or past simple.
The simple past can also be used to describe a habit which stopped in the past. It can have the same meaning as "used to." To make it clear that we are talking about a habit, we often add expressions such as: always, often, usually, never, when I was a child, when I was younger, etc.
I studied French when I was a child. He played the violin. He didn't play the piano. Did you play a musical instrument when you were a kid? She worked at the movie theater after school. They never went to school, they always skipped class.
So let’s take a look at first example:
I studied French when I was a child.
When I say:
I was studying French when I was a child.