If we suffer a breakdown that forces us to drive at an abnormally reduced speed, although we do not seriously disturb traffic, can we drive on the shoulder?

The shoulder is the part adjacent to the carriageway, not intended for the circulation of motor vehicles, except in special cases. In order to use the shoulder for driving, it is essential that it be sufficient and passable and that the MAM of the vehicle is not greater than 3,500 kg.
If we drive in an emergency at an abnormally reduced speed, that is, below the minimum, the shoulder may be used to drive if we seriously disturb traffic; if not, we must drive on the carriageway.