Complex sentence: Refusal to supply can be illegal only, when the company refusing to deal has market dominance and even dominant companies are only required to supply in specific situations where refusing to supply would be abuse of a dominant position.
Split into simpler sentences: Refusal to supply can be illegal only, when the company refusing to deal has market dominance. Even dominant companies are only required to supply in specific situations, where refusing to supply would be abuse of a dominant position.