"Are you already convinced of my standpoint?"
Translation:Bent u al overtuigd van mijn standpunt?
Zitten, staan and liggen are used when it's about position/location, otherwise stick to zijn.
A rule of thumb for the positional to be: When an actual sitting, standing or lying position is involved (for e.g. humans, animals, dolls, robots, etc.) use that one. In other cases: use zitten when it's inside something else (the beer is in the crate = het bier zit in het krat), use staan when it is upright and/or when it can (theoretically) can fall over (car, pole, cupboard, upright book), use liggen when it cannot fall over, or when it already has fallen over (carpet, lawn, fallen over chair, ball, lying book).
There are a lot of exceptions and things that aren't covered by the rules of thumb above. A plate staat next to cutlery that ligt, a building depends on what building and in what context. A ship ligt in the water, but staat on land, etc, etc, etc.