"a" refers to the article as far as I know. I learned to put it before the possessive pronoun, at least I think I remember it that way
Yes...but, if you don't use "a", you are absoluty that is the skirt you wearing....
If you write without the "a" it will sounds more like "about my skirt" and no "over my skirt" so, it will not make sense. For instance: The book is about cats = O livro é sobre gatos.
I dont see this way. For me, using a or not, both show the same meaning.
What is the difference between andar and caminhar? Don't they both mean to walk?
yes, they mean the same. But when you talk about the act of "walking" (like the sport) you use "caminhar". He went walking = ele foi caminhar / ele foi fazer caminhada. But you can also say "ele foi andar", but it sounds lightly more informal in this case.