Learn Spanish (Spain): El restaurante está sucio.

In Spanish, we use the verb estar (which becomes está here) for temporary states or conditions. Being dirty is a temporary state of the restaurant. If we wanted to describe an identity or a permanent characteristic, like saying it is a big restaurant, we would use ser (which becomes es).

Why does sucio end in an -o?

Adjectives in Spanish have to match the gender of the noun they describe. Because restaurante is a masculine noun, the adjective takes the masculine ending -o. If we were talking about a feminine noun, like la cocina (the kitchen), the word would change to sucia.

Why does está have an accent mark on the last letter?

The accent mark shows that the spoken stress is on the final syllable. It is also crucial for telling words apart: está with an accent means "he/she/it is" (from the verb estar), but esta without an accent means "this" (as in esta mesa - this table).

If I already mentioned the restaurant in a previous sentence, how do I say "It is dirty"?

You would simply drop the subject and say Está sucio. Spanish doesn't have a direct subject pronoun for "it", and you don't need one anyway! The verb está already tells the listener you are talking about a third-party "he", "she", or "it".