the correct sentence is " if you are hungry , I will cook for you " . second sentence is wrong because it is obvious that whatever is cooked, is food.

'weather ' in this sentence is singular, and with a singular word we use ' was ' .

in the sentence , subject is the doer or all the doers . in the given sentence friends and the person who is speaking ie. I , are playing, so friends and I is the subject

what is the time in your watch

casual party. because opposite of formal is casual