Final answer:
The sentence 'Juan prefiere esa mantequilla' is grammatically correct in Spanish. The subject is 'Juan' who prefers a specific butter.
Step-by-step explanation:
The sentence 'Juan prefiere esa mantequilla' is grammatically correct in Spanish. The subject of the sentence is 'Juan', which is the third-person singular form of the verb 'preferir'. The verb 'prefiere' agrees in gender and number with the subject. In this case, 'esa mantequilla' is the direct object, which means that Juan prefers that particular butter.
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