Ok so we have been tackling this terribly difficult verb tense to put into practice.
Theoretically we understand it:
2. Repetitive action in the past
3. Started in the past but unfinished
Here is why I think it is so confusing for most people.
- For and since can both be used with the past perfect.
- Since can only be used with perfect tenses, for can also be used with the simple past.
Here is a great video compilation where you can hear it being used in every day situations. Enjoy!