Complex sentence: Motion relative to a rotating frame results in another fictitious force, the Coriolis force; and if the rate of rotation of the frame is changing, a third fictitious force, the Euler force is required.
Split into simpler sentences: Motion relative to a rotating frame results in another fictitious force, the Coriolis force. If the rate of rotation of the frame is changing, a third fictitious force, the Euler force is required.