'when more than one are' is the correct phrase.
I am, you are, he/she/it is, we are, they are.
That is the correct spelling "princesses" for more than one princess.
That is the correct spelling of the word "multiple" (more than one, or a higher number).The correct spelling is multiple (more than one, many).
Yes, hostesses is correct.
that is 'antennae' .
Correct. Two lines never meet in more than one point.
Parenthesis is for one and parentheses is for more than one.
depends if it is more than one
One vertebraTwo or more vertebrae(Vertebrae is the more correct but vertebras is still acceptable)
Either is correct, although in some contexts one may be more common than the other.
Ally is the singular form. Allies is plural, (more than one).
More than one province is "provinces".