'for' to express length of time

You are quite right, Serj. We use for as a preposition when we are talking about a period of time:

• For a few years my brother worked on the plant.
• My brother worked at the factory for a few years.

We don't know exactly when it was and I don't know how old your brother is but it might have been in the 90s, the 80s or the 70s or even earlier and it lasted for two or three years.

For can be used to describe a period of time in the past, present or future:

• The English course that I'm attending lasts for three months.
• Then I shall be on holiday in Dublin for five days.
  Last year I went to Australia and stayed for six weeks.

However, if you use for with the present perfect or present perfect continuous tense, it indicates a period of time which started in the past and continues up to the present time:

• My sister has worked as a vet for fifteen years now - since 1987.
• Those oak trees have been standing in Greenwich Park for centuries - since the 18th Century, I think.

Note that since is used to indicate the starting point of the action and for measures the period of time up to the present.

during to express length of time

We also use during as a preposition when we are talking about a period of time, but the meaning is different. During means (at some point) in the course of. Compare the following:

• I saw not one duck on the lake during the whole of last summer.
• I don't know when exactly but he must have left during the night.
• I expect he'll phone me at some stage during next week.
• It must have rained here during the last fortnight as the ground is quite soft and damp.

Remember the difference by thinking that during tells us when something happens, for tells us how long it lasts.

During does not work in your original sentence, Serj, because it introduces a subordinate clause starting with when or that and the sentence is incomplete. There is no main clause.

• During the few years…
• During the few years (when / that) my brother was working in the factory…

If we add a main clause, the sentence will be complete and grammatically correct.

• During those years (when / that) my brother was working in the factory, I was studying at university.

Note that we can replace during…when or during…that with the conjunction while:

• My brother was working on a farm while I was studying for my masters degree.
• While my brother was at home working on the farm, I was away at university.

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