本学期的一些主要语法项目介词短语介词短语表示时间(for, since,at)People have sent Christmas cards to each other for about 150 years.(时间段)I have lived here since I was born .(时间点)at 5 o’clock(时间点) at Christmas(时间段)用介词短语描写物品I prefer the one with the white collar.以wh-引起的特殊疑问句What do people usually do at Christmas?Why has electricity become so important in our lives?Where do we usually find this sign?一般现在时的用..