How does a magic circle works?Is it like a threshold?
Can it be broken by physical means?I have read the books but I can't remember how it was used.The only example I remember is when the frog demon(Storm Front) attacked Dresden in his house.Why the demon couldn't drop a chair to break the circle in the basement in which Dresden and Susan have found shelter?
It is like a threshold, though how much so I am not sure, there's a lot of people who know more about that than I do. but for the final question (And note that this isbased on some passages in the novels, and not Rules from the RPG books), circles like that can only be broken by Mortal acts, So the Demon couldn't get through. Also, in the same book, Harry broke Victor Sells' circle by throwing a film container across it.
Is it possible for a circle to be broken by a throwing object?For example a demon thowing a chair to the occupants of the circle.
No - at least not in the setting as written.
There are many spots where Dresden mentions that a being that can be imprisoned in a circle cannot throw something through it but anyone with freewill can - and that breaks the circle.
Spoilers for Turn Coat:
The encounter with Binder in Turn Coat comes to mind. Surrounded by summoned things that can't cross a threshold the circle is safe, but their summoner (Binder) could break the circle. And if Murphy shot from the circle, that would break the circle.
Then every summoned being is treated as if it had just crossed a threshold because a circle was established around them - destroying them.
And yes - there are a few things in the early books that break the rules here (Bob tossing a potion through an active magic circle) but the first three books... well, they have consistency problems (including how one of the characters changed her name between her appearance in book 1 and discussion about her in book 3). There are WoJs out there to effect of "Oops - I missed a few things back then".
Then there are the problems that editors created. In my copy of Grave Peril it clearly says that the While Council was the group behind Stroker - but later printers have it corrected to White Court.
Richard