1.jill is lreland now.She arrived there three
days ago.
(She has been there for three days)
2.They are married.They got married six months ago.
They've(married for six months.)
3.Liz is studying medicine at university.She started three years ago.
She has( started since three years ago.)
4.Dave plays the piano.He stared when he was seven years old.
Dave has(started since he was seven years old.)
究竟是用for 还是since 关键是看前面的动词是一个可持续性的,还是短暂性的.比如说has been Ireland,在爱尔兰,显然是一个可持续性的动作,可以用for+时间段；而started “开始”只能是一个瞬间发生瞬间完成的动作,需要用since+时间.