Final answer:
The sentence 'Juan prefiere esa mantequilla' is grammatically correct in Spanish.
Step-by-step explanation:
The sentence 'Juan prefiere esa mantequilla' is grammatically correct. In Spanish, the subject of a sentence is often implied and can be understood from the context. In this sentence, 'Juan' is the subject and 'prefiere' is the verb, meaning 'Juan prefers' or 'Juan likes'. 'Esa mantequilla' means 'that butter', indicating the object of Juan's preference.
Learn more about Sentence structure in Spanish