Learn Spanish (Latin America): Las sábanas están sucias, quiero lavarlas. - The sheets are dirty, I want to wash them.

las is the direct object pronoun for them (the sheets). When you have a conjugated verb followed by an infinitive (like quiero lavar), you have two choices for where to put this pronoun. You can attach it directly to the end of the infinitive (quiero lavarlas) or put it right before the conjugated verb (las quiero lavar). Both are perfectly correct and mean the exact same thing!

How do we know to use las instead of los for the pronoun?

Because pronouns have to match the noun they are replacing. The pronoun refers back to las sábanas, which is a feminine and plural noun, so the pronoun must also be feminine and plural (las).

Why do we use están instead of son?

We use a form of estar here because being dirty is a temporary physical condition or state of the sheets right now. If we used ser (son sucias), it would sound like being dirty is a permanent, defining characteristic of the sheets themselves.

Why is the adjective sucias and not sucio?