1.B
此题考查全部倒装.
如果句子主语为名词(短语),谓语动词为不及物动词,可以把做句子状语的副词/介宾提前至句首,再加谓语,最后加主语.
正常句序是:The teacher came in,倒装成In came the teacher.
2.B
固定短语imagine sb/sth/one's doing sth,it即sth,happening即doing sth.
选A的话,则使用宾语从句imagine (that) it would happen,“想象它将会发生”,句意不通.
3.原因很简单,句子的主语形式不同.
如果句子主语是名词(短语),则使用全部倒装,为Here comes the bus/teacher;
如果句子主语是代词,则主语谓语不倒装,即Here you are./ Here it is.