6. at a time的意思是“一次”。
at times相当于from time to time，意思为“时常，不时”；
at all times的意思是“总是”。
如：He gets angry with his son at times. 他时常对他的儿子生气。
He is willing to help his friends at all times. 他总是愿意帮助他的朋友们。
He is a good speaker, and is able to make a speech for two hours at a time. 他是个很好的演讲者，并且能够一次做一个两小时的演讲。
 7. through, across
through意为“穿过，通过”，着重从空间穿过；
across也为“穿过”，但指从平面的一端到另一端或十字交叉。
如：The train is running through the tunnel. 火车正从隧道中穿过。
When you see the bank, walk across the road. The post office is right there. 当你看到银行时，走过马路。邮局就在那儿。
 8. across, over
across和over：这两个单词都可以用在“过桥”时，walk across/over the bridge，
over还可以表示“越过”，climb over the hill, jump over the chair, fly over。