Final answer:
The sentence contains a grammatical error in Spanish language.
Step-by-step explanation:
The sentence "La silla es tan duro como la mesa" is grammatically incorrect in Spanish.
"Duro" is an adjective that agrees in gender and number with the noun it describes. Since "silla" is a feminine noun, the correct form of the adjective is "dura". Therefore, the correct sentence would be "La silla es tan dura como la mesa".
Keywords: Spanish grammar, grammatical agreement, adjective agreement
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