Continuity of actions
Continuity shows progression of actions or states. An action in progress is continuous.
When we came, she was singing. (when we came, there was an action in progress)
When (after) we came, she left
When we came, there was no action in progress.
When we came, she was leaving. (When we came, there was an action in progress)
We are singing now (there is an action in progress)
We sing and dance every day (there is no action in progress)
Basically, in progress means at a given time. At a given time equals now, then, at this moment, at that moment, after 2 minutes, tomorrow at 5, on Thursday between 7 and 9 pm etc. I call them precise time progress. We may use less explicit time periods like tomorrow, next week, next month and even next year. The same applies to the past and the present. Last / this year, last / this month, yesterday / today
Last year he was working on another project. This year he is working on this project.
Let me talk about states in progress.
You are being naughty. (a state of being naughty is in progress)
A little tricky example.
When we came, she was pale.
Here the verb be is not in the continuous form. Although we might think that her state was in progress, we don’t want to stress this progress. We just want to state the fact that her face was pale.
He was naughty. (no state in progress, just the fact)
He was being naughty. (there is a state in progress. We want to stress the progress. Usually, he is not naughty, but at that time he was.)
We normally express continuity of actions by using continuous / progressive tenses. The verb takes the ing ending. We apply other rules of tense formation there.
So, as I have figured it out, continuity of an action, in light of tense formation, has to have 2 key aspects.
1. Action in progress 2. a period of time in the frame of which this action takes place.
This period of time can be really long.
I have been doing this for ages. We have been working here for 10 years
Or it can be short.
I have been doing it for 5 minutes.
Or this period of time can be so short that it shrinks to a point in time, like now, then, at 5 pm etc.
We are singing now. We were dancing in the hall at 5 pm. We will be dancing at the presentation at 5 pm tomorrow.