12已经发生多久了 When/How long & For/since

提问：
When+一般过去时
How long +现在完成（进行）时
A：when did it start raining?
B: It started raining an hour ago/ at 1 o'clock.
——
A:How long has it been raining?
B:It's been raining for an hour/ since 1 o'clock.
——
A:When did Joe and Carol first meet?
B:They first met a long time ago /when they were at school.
——
A:How long have Joe and Carol known each other?
B:They've known each other for a long time/ since they were at school.

For + 一段时间（ two hours, a week, 20 minutes,15years , five days, a long time, six months，ages...）
Since + 某个时间点（ 8 o'clock ,1977, Monday, Christmas ,120 May, lunch time ,April ,they were at school...）

For:
陈述句可以省略:
Then they married （for ）ten years.
否定句不可以省略:
They haven't had a holiday for ten years.
后接all...，前面不可以用for :
I've lived here all my life.

强调某件事情发生你现在的时间用以下句型：
It's (a long time,two years etc.) since something happened:
It's two years since I last saw Joe.
It's ages since we went to the cinema.
提问： How long is it since you last sae Joe?
How long is it since Mrs. Hill died?