Complex sentence: Along with a group which was intended to prepare the ground for the Duke's wedding to Isabella of Portugal, van Eyck's task was to paint the bride, so that the Duke would be able to form a picture of Isabella before the marriage.
Split into simpler sentences: Along with a group which was intended to prepare the ground for the Duke's wedding to. Van Eyck's task was to paint the bride, so that the Duke would be able to form a picture of Isabella before the marriage.