Learn French: La cuisine est sale, il va la nettoyer. - The kitchen is dirty, he is going to clean it.

In French, pronouns must match the gender of the noun they replace. Since kitchen is feminine (la cuisine), we use the feminine direct object pronoun la to mean "it".

Why is la placed right before nettoyer and not before va?

In sentences with a conjugated verb and an infinitive (an unconjugated verb like nettoyer), the object pronoun goes right before the infinitive it belongs to. Since he is cleaning "it", la goes directly in front of nettoyer.

Does the word sale mean the kitchen is on discount?

No, sale is a false friend (a word that looks like English but has a totally different meaning). In French, sale simply means "dirty". If you want to talk about shopping discounts, you would use the word les soldes.

How do we know il va nettoyer means "he is going to clean"?

This uses a very common structure called the near future. Just like in English, you conjugate the verb aller (to go) and add an infinitive verb. So il va (he is going) + nettoyer (to clean) gives us "he is going to clean".