1.
You use ~ to emphasize that something is only just true or only just the case.
He could ~ breathe...
I ~ knew him...
He was ~ more than a boy...
ADV: ADV before v, ADV group, oft ADV amount emphasis
2.
You can use ~ to say that something is not true or is not the case, in a humorous or critical way.
It can ~ be coincidence...
= hardly
ADV: ADV before v, ADV group
3.
If you say ~ had one thing happened when something else happened, you mean that the first event was followed immediately by the second.
Scarcely had they left before soldiers arrived armed with rifles...
ADV: ADV before v