07 - "Since", "for", "ago" and "during"

"Since": is used with a moment in time, and generally takes the present perfect (it may sometimes take the past perfect, but not the present simple, nor the past simple), eg:

- Motor transport has developed massively since the start of the 20th century. (present prefect simple - describing a state of being which began in the past and continues into the present).
- I have been working hard since 9:00 am. (present perfect continuous - used for a more immediate, on-going action which started in the past and is continuing in the present, or still has an impact on the present).

"For": is used with a duration of time and may be used in the past, the present or the future, eg:

- Mitterrand was President for 14 years.
- The seminar will last for 10 weeks.
- We hope to go on holiday next summer for three weeks.

"Ago": is used when indicating how much time has passed between an action and the present.  It very clearly situates the action in the past, and takes the past simple, eg:

- Dinosaurs became extinct about 60 million years ago.
- She telephoned five minutes ago.

"During": is not used to introduce a length of time, i.e. you cannot say "during 10 minutes", "during three weeks", etc.  Instead, "during" situates an action within a recognised period of time, e.g.: "during the summer", "during the 20th century", "during the holidays", etc.