Learn Spanish (Latin America): ¿Acabas de firmar el acuerdo?

The phrase acabar de followed by an infinitive (like firmar) is a set expression used to say that you just did something. Even though the action happened in the recent past, Spanish uses the present tense of acabar for this. Acabas de firmar translates directly to "you just signed."

Can I leave out the de and just say acabas firmar?

No, the de is completely required. The formula for this recent past expression is always acabar + de + an unconjugated verb. Without the de, the phrase is grammatically incorrect, similar to dropping the "to" in the English phrase "going to do something."

Is el acuerdo related to the phrase de acuerdo?

Yes! El acuerdo is a noun that means "the agreement" or "the deal." You probably already know the common phrase estoy de acuerdo (I agree), which literally translates to "I am in agreement."

Does firmar mean to make something firm?

No, firmar specifically means "to sign" a document, letter, or agreement. If you want to talk about a physical signature, you use the noun la firma.