当表时间,地点的副词here,there,in,out,up,down,now,then,away,off及表地点的介词短语位于句首,主谓变谓主。A shy girl was sitting at the back of the room.
3)不定代词each,one,another,something,somebody,nothing,nobody,everyone,everything,anyone,anything等作主语时,谓语动词用单数形式。