Final answer:
The appropriate irregular conditional form of the verb poder is podrían.
Step-by-step explanation:
The appropriate irregular conditional form of the verb poder to fill in the blank in the sentence is podrían. The sentence would then read: Paula y Carlos podrían ir al cine con nosotros, si no tuvieran ya otros planes. In this sentence, the verb poder is conjugated in the third person plural for the conditional tense.
Learn more about Irregular conditional verb form in Spanish