VOA慢速英语:For Youths in US, a Jump in Media Use

not necessarily未必，不一定

例句：Clouds do not necessarily forecast rain.
      云层未必预示下雨。

2.You're really switching back and forth sequentially from different tasks, just doing it rapidly, and that you don't really do either task as well as you would do them if you did them one at a time."

back and forth来来往往地,来回地

例句：The shuttle flies back and forth on the loom.
      织布机上梭子来回飞动。

3.About one-fourth of those who used media the least reported that their grades were mostly average or below. But that was true of half the heavy media users.

that was true of这里是省略句。前面一个句子和后面一个句子说的是同一件事，只是不同的主体。that was true指的是used media the least reported that their grades were mostly average or below。

4.Children who had any rule limiting the use of any kind of media were exposed to an average of about three hours less media a day.

expose to使易受, 使受

例句：They have exposed the enemy's plot to the light of day.
      他们已把敌人的阴谋暴露在光天化日之下。

5.It looks like the girls just sort of lose interest in the games and they drop off and start doing other things, whereas the boys' interest remains strong.

sort of有几分地

例句：Most nations of the globe have some sort of color prejudice.
      世界上多数国家多少都有些种族偏见。

drop off下降,减少

例句：Yen dropped off.
      日元下跌。