Final answer:
The given sentence is grammatically incorrect in Spanish.
Step-by-step explanation:
The given sentence, 'La silla es tan duro como la mesa', is grammatically incorrect.
In Spanish, adjectives must agree in gender and number with the noun they modify. Since 'silla' is a feminine singular noun, the correct form of the adjective 'duro' (hard) should be 'dura'.
The correct sentence would be 'La silla es tan dura como la mesa'.
Learn more about Grammar in Spanish