Learn Hardcore Spanish (Latin America): La firma debe estar en la última línea del contrato. - The signature must be on the last line of the contract.

Debe is the third-person singular present indicative of deber. In this context it expresses a formal obligation or necessity, similar to English “must”. So La firma debe… means “The signature must…”

Why is the verb estar used here instead of ser?

In Spanish, estar is used to indicate location or position, whereas ser describes inherent qualities or identity. Since we’re talking about where the signature goes (its physical placement), we use estar:

correct: estar en la última línea
incorrect: ser en la última línea

What is the role of the preposition en in en la última línea? Could I use sobre instead?

En indicates that something is located on or within a defined space—in this case, the line. You could say sobre la última línea, which also means “on the last line,” but en is more common for precise placement. Sobre sometimes implies “on top of,” which can feel slightly less specific.

Can we replace línea with renglón?

Yes. Línea and renglón both mean “line (of text).”

línea is more universal and used in many Latin American countries.
renglón is also correct but less common in some regions.

Why is última feminine, and how does gender agreement work here?

Adjectives in Spanish agree in gender and number with the nouns they modify. Línea is feminine and singular, so última must be feminine singular as well. If it were el último párrafo, último would be masculine.

What does del stand for in del contrato, and why is it needed?

Del is a contraction of the preposition de + the definite article el. It literally means “of the”. Here it shows possession or association: la línea del contrato = “the line of the contract” (i.e., the contract’s line).

Could we say La firma tiene que estar en la última línea del contrato instead?

Yes. Tener que + infinitive also expresses obligation. The nuance is:

debe sounds more formal or impersonal (“must”).
tiene que is a bit more conversational (“has to”).

Is the definite article la before última línea mandatory?

Yes. In Spanish we normally include the definite article when specifying a particular element:

correct: en la última línea (“in the last line”)
incorrect: en última línea

Could I rephrase it as La firma debe ir en la última línea del contrato?

Absolutely. Ir here acts like “to go” or “to be placed.”

debe estar = “must be (located)”
debe ir = “must go (there)”
Both convey the same requirement, with only a slight stylistic difference.