Learn Spanish (Latin America): El gerente dice que eso no es lo que prometió. - The manager says that is not what he promised.

In this sentence, lo que translates to "what" or "that which." We use it to refer to an abstract idea or an unnamed thing—in this case, the unspecified thing that was promised. You can't use qué because it isn't a direct or indirect question, and you can't use just que because there is no specific noun right before it for it to refer back to.

Why does the sentence say dice que eso instead of just dice eso?

In English, we often drop the connecting "that" and just say "The manager says that is not what he promised." In Spanish, you can never drop the conjunction que. Therefore, we need que to connect the two clauses, and eso to act as the subject of the second clause, literally meaning "says that that (thing) is not..."

Why is the word for "that" eso here, rather than ese or esa?

We use the neuter pronoun eso when referring to a general concept, a situation, or an unnamed statement rather than a specific masculine or feminine noun. Here, the manager is pointing out that a general situation or proposition is not what he agreed to.

Why is prometió in the preterite tense rather than the imperfect (prometía)?

The preterite is used here because the act of making the promise is seen as a single, completed event in the past. He made the promise at a specific moment, rather than it being an ongoing, continuous, or habitual action in the background.