Где употреблять was а где were

I wasYou wereHe was She was It wereWe wereThey wereWas wereHe was a doctor.I was in this country.That city was beautiful.The book was interesting.The boy was swimming. Mary was in the park.I was playing the piano.They were doctors.We were in this country.Those cities were beautiful.The books were interesting.The boys were swimming.Mary and Tom were in the park.We were playing the piano.