Put the remaining food into the bin. **** This is correct *** Be careful not to hurt your neck. **** This is correct *** (told to the children who are hanging downward from a bar) Watch out where you're going! ****This is correct **** (told to a child who is running and nearly bumps into another child) **** So, all three of your sentences were correct. But some of your comments in parenthesis needed to be changed. I hope this helps!

Be careful not to hurt your neck is fine ! we would say in spoken English ' Watch where you're going !' or you could just say ' watch out!'.