In the book English Grammar In Use by Raymond Murphy they gave the example for Past Continuous - ´Today Helen is wearing a skirt. Yesterday she was wearing trousers´. Please explain to me why it isn´t Past Simple because the action is complete, it had finished.
The action today isn't finished yet and is expected to continue for the rest of the day.
Yesterday she was wearing trousers for the whole day, not just part of it.
Today Helen wears a skirt for the photoshoot. Yesterday she wore trousers.
This is for a limited period of time (during the photoshoot) not all day.
However, I would be quite happy to say "Today Helen is wearing a skirt. Yesterday she wore trousers." idiomatically.
Sometimes you have to learn rules first, and then you will learn later that rules are broken.
Very simple: was wearing is the past of is wearing. Also, it means over a period of time in the past.
Of course, you can say: She wore trousers yesterday.
I am studying today. Yesterday, I was not studying. In fact, I didn't study at all yesterday. I simply haven't studied at all recently.
Continuous forms are for what a person wants to say as an ongoing action. The speaker chooses what tense to use in these cases.