Questions & Answers about Við höfum verið að bíða síðan í morgun.
Because the sentence describes an action that started earlier and is still continuing now.
- Við bíðum = We wait / We are waiting in a general or simple present sense.
- Við erum að bíða = We are waiting right now.
- Við höfum verið að bíða = We have been waiting.
So höfum verið að bíða is the best match for the English have been waiting.
In this pattern, vera að + infinitive is used to show that someone is in the middle of doing something.
So:
- vera að lesa = to be reading
- vera að borða = to be eating
- vera að bíða = to be waiting
When you put that into the perfect, you get:
- hafa verið að bíða = to have been waiting
So the að is part of a common verbal construction, not a separate word you would translate literally by itself here.