USAGE NOTE 用法说明: ago, for, and 和 since
Use ago to show how far back in the past something happened. Always use it with verbs in the simple past tense. ago 用于表示某事在多久以前发生，总是与动词的简单过去式连用:
I came here a year ago. 我一年前来到这里。
Use for and since with the present perfect tense to talk about a situation that began in the past and continues now. Since is used with dates and times. for 和 since 与现在完成式连用，用于表示一个状况从过去开始并延续到现在。since 与日期和时间连用:
I've been here since 1997. 自1997年以来，我就在这里了。
She's been waiting since 12 o'clock. 她从 12 点开始一直等到现在。
For is used with periods of time. for 与一段时间连用:
I've lived here for six years. 我在这里住了六年。
She's been waiting for three hours. 她已经等了三小时。