Final answer:
The correct verb form to fill in the blank is 'sepas'.
Step-by-step explanation:
The correct verb form to fill in the blank in the sentence is sepas. The sentence would then read: Yo te recomiendo que sepas el poema de memoria. The verb saber is used in the present subjunctive form to indicate a recommendation or suggestion. In this case, the recommendation is for the person to know the poem by heart.
Learn more about Verb forms in Spanish