例如：I have lived here for more than twenty years.我住在这儿二十多年了。
I have lived here since I was born. 我从出生起就住在这儿了。
I worked here for more than twenty years. （我现在已不在这里工作。）
I have worked here for many years.（现在我仍在这里工作。）
1）（对） Tom has studied Russian for three years. = Tom began to study Russian three years ago, and is still studying it now.
2）（错） Harry has got married for six years. = Harry began to get married six years ago, and is still getting married now. 显然，第二句不对，它应改为 Harry got married six years ago. 或 Harry has been married for six years.
1） since +过去一个时间点（如具体的年、月、日期、钟点、1980, last month, half past six）。例如：I have been here since 1989. 1989起，我一直在这儿。
2） since +一段时间+ ago。例如：I have been here since five months ago. 我在这儿，已经有五个月了。
3） since +从句。例如：Great changes have taken place since you left. 你走后，变化可大了。 Great changes have taken place since we were here. 我们走后，变化可大了。
4） It is +一段时间+ since从句。例如：It is two years since I became a postgraduate student. 我考上研究生有两年了。