Learn Hardcore Spanish (Spain): El barrio cuya plaza decoramos mañana tendrá una fiesta enorme. - The neighborhood whose square we decorate tomorrow will have a huge party.

Cuyo is a possessive relative pronoun meaning “whose.” It links el barrio (the neighborhood) to la plaza (the square that belongs to it). A simple que would only introduce a clause (“the neighborhood that…”), without expressing possession.

What does cuyo agree with: the possessor or the thing possessed?

It agrees with the thing possessed. Here, plaza is feminine singular, so we use cuya. The possessor (barrio) is masculine singular, but cuyo never agrees with the possessor—only with the noun immediately following it.

Why isn’t there a de before cuyo if it implies “of which” or “of whose”?

Cuyo already incorporates the preposition de (or whichever preposition marks the possession). Adding another de (e.g. de cuyo) would be redundant and ungrammatical.

Why is the verb decoramos in the present tense when it refers to a future action (“tomorrow”)?

Spanish frequently uses the present indicative for scheduled or near-future events. Saying decoramos mañana is equivalent to “we’re decorating tomorrow.” You could also use:

decoraremos mañana (future tense), or
vamos a decorar mañana (ir a + infinitive)
—all convey similar meaning, with minor stylistic differences.

Is it okay to place mañana at the end of the clause? Could it go somewhere else?

Yes. Temporal adverbs like mañana are flexible:

Mañana decoramos la plaza (emphasizes “tomorrow” at the start)
Decoramos la plaza mañana (neutral placement)
Both are correct. Positioning depends on what you want to highlight.

Could the adjective enorme appear before fiesta? Would that change the meaning?

You can say either:

una fiesta enorme (default)
una enorme fiesta (more emphatic or stylistic)
The basic meaning (“a very big party”) stays the same, but pre-noun placement often feels more expressive.

Why isn’t there a subject pronoun before decoramos (e.g. nosotros decoramos)?

In Spanish, subject pronouns are usually omitted because the verb ending already indicates person and number. Including nosotros is not wrong, but it’s redundant unless you want to emphasize who is doing the decorating.

Why is it una fiesta enorme instead of la fiesta enorme?

Una is an indefinite article, signaling that the party is not previously identified in the conversation. Using la (definite) would imply both speaker and listener already know which specific party is meant. Here, it’s simply “a huge party” that the neighborhood will have.