Short form adjectives

[david-us.livejournal.com]

When telling a girl that she is pretty, which would be more correct?
"Tы очень красивая" or "Ты очень красива"

Are both sentences grammatically correct? If so, is there any differences in their meaning?

Basically, I'm wondering how one knows to use the short form of an adjective and when to use the long form.

I have always been taught that the long form should be used if it is modifying a noun which immediately follows it; for example:
"Ты очень красивая девушка."

Finally, a somewhat unrelated question:

Let's say you are trying to tell a woman that she is very beautiful, but you are still trying to be formal and polite. Which one of the following sentences would be correct?

Вы очень красивая
Вы очень красива
Вы очень красивые
Вы очень красивы

David Emerling
Memphis, TN