When...? How long...? For and Since

When…? Is used in past simple and How long…? Is used in present perfect.

Examples:

A             :  When did it start raining?

Q             :  It started raining (2 hours ago/at 12 o’clock)

A             :  How long has it been raining?

Q             :  It’s been raining (for an hour/since 12 o’clock)

A             :  When did John and Sue firs meet?

Q             :  They first met (a long time ago/when they were child)

A             : How long have John and Sue known each other?

Q             : They have known each other (for a long time/since they were child)

We use both for and since to say how long something has been happening.

For is used when we say a period of time (an hour ago/2 hours ago/5 weeks/1 month/1 year/etc). And we use since to express the time of the period is started (5 o’clock, Monday, 1989, etc).

Give your attention to the examples below:

Tom has been working in bank for 1 year.

 (not since 1 year)

Sally has not seen Tom for three days.

Tom has been living here since 1989.

(from 1989 until now)

Sally has been seen Tom since Monday.

(from Monday until now)

For can be left out but not usually in negative sentences

Example               :  They have been married (for) ten years.

                               They have not had a holiday for ten years. (in negative sentence, you must use for)

We say It’s (a long time/five years/etc) since something happened

Example               :  It’s five years since I last saw Tom (I haven’t seen Tom for five years)

The question is How long is it since…?

Example               :  How long it since you last saw Tom? (when did you last see Tom?)