Final answer:
The sentence 'La silla es tan duro como la mesa' is grammatically incorrect because the adjective 'duro' should agree in gender and number with the noun it modifies.
Step-by-step explanation:
The sentence, 'La silla es tan duro como la mesa,' is grammatically INCORRECT.
The adjective 'duro' needs to agree in gender and number with the noun it modifies. Since 'silla' is feminine, the correct form of the adjective is 'dura'.
The correct sentence should be 'La silla es tan dura como la mesa.'
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