Hi..I am still rowing and rowing in grammar

and learning is fun, especially here,with you guys.
1) I'm being foolish.
As I understood we have two rules to tell about the above form :
* It's present continuous, or
*It's because we might use be in the continuous form only with some adjectives like bad,good,foolish,etc.
Am I right here ?
2) How do you like the two sentences:
_ You are being funny ?
_ You are joking ?
3 ) A mother is talking to her son at dinner and reminding him about his previous behaviour with his uncle :
_ If you just were being polite. ( she means if you at least pretended )
_ If you were just polite.
which one is right?
4) Which is the correct to say :
_ How much is you hat ?
_ What does this hat cost ? ( I didn't get it here )