All of us have to stay in the classroom because （后加of,因为后面不是完整句子,所以用固定形式——because of）the heavy rain outside
Don't touch the snake .It is danger（danger改dangerous,它很危险,危险作形容词用)
After drink some milk ,I felt even hungrier.(drink改drinking,after是介词,介词后一般加ing形式)