the teacher told us that pollution (is) a serious problem in the world.为什么括号里不能改成had been

Our teacher told us ______in the hallways. The teacher told us something important yesterday The teacher told us the moon _____around the earth. the teacher told us to h___________in the exercises after class The teacher told us that the earth_(be) round the teacher told us that honsty ---(be)the best policy Yesterday our teacher told us the earth_______(gYesterday our teacher told us the earth_______(go) around the sun our teacher told us earth______(move)round the sun The teacher told us _____[arrive] on time The teacher told us time___________(wait) for no man The teacher told us ________(not be) late for school. The teacher told us to be more___(carefully) The teacher told us __( not be )late again. The teacher told us light ___ fast than sound.(travel) The teacher told us when------ (finish) our homework the teacher told us how we should learn englishell! Our teacher told us that the movie was worth The teacher told us funny joke.改写为感叹句