Complete the sentence with the past-simple present-regret form, not the separate could construction for limited possibility or ability: I understand very little marine biology now, and I regret that present state. If only I ___ about it. (know more)

Explication

Knew more uses the requested past form to represent the speaker's unreal present state of knowledge. Could know more is also natural when if only laments a limited possibility or ability to gain knowledge, but the prompt explicitly asks for the state-focused pattern.