Learn Hardcore Spanish (Latin America): Mi oponente hoy es una niña de diez años que juega muy bien. - My opponent today is a ten-year-old girl who plays very well.

ser (here: es) to identify or define what someone is (profession, role, identity, classification).
estar (here: está) for states, conditions, locations, and ongoing actions with gerunds.

In this sentence, you’re identifying the person’s role:

Mi oponente hoy es una niña de diez años…
= My opponent today is a ten-year-old girl…

Any time you say “X is my friend / teacher / opponent / boss / doctor…”, Spanish uses ser, even if the situation is temporary.

Using está here (Mi oponente hoy está una niña…) would be incorrect. You could say something like:

Hoy está de oponente = Today she is acting as (serving as) an opponent

…but that changes the structure and the nuance.

Why is the word order Mi oponente hoy es… and not Hoy mi oponente es… or Mi oponente de hoy es…? Are these other options correct?

All three options are grammatically correct, with slight differences in emphasis:

Mi oponente hoy es una niña…
- Literally: My opponent today is a girl…
- hoy is an adverb modifying the whole idea. Neutral, natural word order.
Hoy mi oponente es una niña…
- Literally: Today my opponent is a girl…
- Emphasis slightly shifts to hoy (today): you’re foregrounding “today” as the topic.
Mi oponente de hoy es una niña…
- Literally: My opponent of today is a girl…
- de hoy acts like an adjective modifying oponente.
- Very common in sports/competition contexts in Latin America when talking about that day’s rival, match, etc.

In practice, in Latin American Spanish you’ll hear Hoy mi oponente es… and Mi oponente de hoy es… very often. Your original sentence is also fine and natural.

Why do we say una niña de diez años and not something like una niña diez años?

Spanish normally expresses age attached to a noun with:

de + number + años

So:

una niña de diez años = a ten-year-old girl
un hombre de treinta años = a thirty-year-old man
un niño de cinco años = a five-year-old boy

You can’t drop the de here; una niña diez años is incorrect.

Colloquially, you can sometimes drop años and just say:

una niña de diez = a ten-year-old girl (very conversational)

But the de stays.

Why is it de diez años with años in plural?

Because in Spanish, numbers greater than 1 require plural for countable nouns.

1 year = un año
10 years = diez años

So:

una niña de un año = a one-year-old girl
una niña de diez años = a ten-year-old girl

Using de diez año would be incorrect; you must pluralize años after diez.

Could we say tiene diez años instead of de diez años? What’s the difference?

Yes, but the structure changes:

Es una niña de diez años.
= She is a ten-year-old girl. (Age as an adjective-like description attached to girl.)
Es una niña. Tiene diez años.
= She is a girl. She is ten years old.

So:

de diez años modifies the noun niña directly.
tiene diez años is its own clause, with tiene (has) as the verb.

Both are normal and very common. Your original sentence prefers the more compact “noun + de + age” structure.

Why is the relative pronoun que used in que juega muy bien, and not quien or la que?

In Spanish, que is the default relative pronoun for both people and things, especially in restrictive clauses (those that define which person/thing you mean).

una niña de diez años que juega muy bien
= a ten-year-old girl who plays very well
(que refers to niña here.)

Using quien:

quien is mainly used:
- for people
- after prepositions (la persona con quien hablé)
- or in non‑restrictive clauses set off by commas:
  - Mi oponente, quien juega muy bien, es una niña…
    (extra information, not specifying which opponent)

Using la que:

la que is also possible but sounds more formal and tends to be used:
- after prepositions: la niña con la que juego
- or when you want to emphasize or contrast:
  - La que juega muy bien es una niña de diez años.
    (The one who plays very well is a ten-year-old girl.)

In your sentence, que is the most natural and usual choice.

Why is it que juega muy bien (simple present) instead of something like que está jugando muy bien?

juega (simple present) in Spanish often covers both:

general ability / habit: she plays very well (in general)
commentary about a current game / sport, especially in narration.

So:

que juega muy bien can mean:
- She generally has a high level of skill.
- Or, in a match commentary style context, she’s playing really well (and this is the known fact about her).

If you wanted to focus specifically on how she is playing right now, with extra emphasis on the ongoing action, you could say:

…una niña de diez años que está jugando muy bien (hoy).
= …a ten-year-old girl who is playing very well (today).

But the original sentence reads more like a description of her ability / usual level, not just today’s performance.

Why do we say muy bien and not muy bueno or muy buena?

Because here we are describing how she plays (the manner of the action), not what kind of thing she is.

bien is an adverb: it describes verbs (how someone does something).
bueno / buena are adjectives: they describe nouns (what someone/something is like).

So:

juega muy bien
= she plays very well (adverb modifying juega)

If you said:

Es muy buena.
= She is very good. (adjective describing she)

You can combine both:

Es una niña de diez años que juega muy bien y es muy buena.
= She’s a ten-year-old girl who plays very well and is very good.

But after juega, you need bien, not buena.

What form of the verb is juega, and what should I know about jugar?

juega is:

3rd person singular
present indicative
of the verb jugar (to play, usually games/sports, not instruments)

Key points about jugar:

It’s a stem-changing verb (u → ue) in the present tense (and some other forms):
- yo juego
- tú juegas
- él / ella / usted juega
- nosotros jugamos (no change here)
- ustedes / ellos juegan
Spelling:
- Infinitive: jugar (g + a)
- 1st person preterite: jugué (insert u to keep the hard /g/ sound before e).

In your sentence:

que juega muy bien = who plays very well
The subject (she) is understood through the verb ending; Spanish doesn’t need to add ella.

How does gender work with oponente here? The word doesn’t change, but the girl is female.

The noun oponente is one of those that has one form for both genders:

el oponente = the (male) opponent
la oponente = the (female) opponent

In your sentence:

Mi oponente hoy es una niña…

We don’t see an article with oponente (like el / la), so oponente itself doesn’t show gender.
The predicate una niña makes it clear the opponent is female.

If you added an article, you’d show gender with the article:

Mi oponente hoy es una niña… (context tells us she’s female)
Mi oponente hoy es un niño… (then we’d know it’s a boy)

But oponente itself doesn’t change form between masculine and feminine.

Why is it niña and not something like chica or muchacha? Are those different?

All three can refer to a girl, but they have slightly different nuances and regional preferences:

niña
- Literally: girl / little girl / child (female)
- Strongest association with childhood / being a kid.
- In Latin America, often used for younger girls (children), though it can stretch up to 11–12 or so depending on context.
chica
- Very common in Spain; also widely understood in Latin America.
- In much of Latin America, it often suggests a girl/young woman a bit older than a small child (preteen, teenager, young adult).
muchacha
- Common in many parts of Latin America (e.g., Mexico, Central America, the Caribbean).
- Often means teenage girl or young woman; in some regions can also mean “maid / domestic worker,” depending on context.

In una niña de diez años, niña fits perfectly because 10 years old is very clearly still a child, not an older teen or adult.

Why is the preposition de used in de diez años? Could we ever say a los diez años instead?

de here links the age to the noun as a characteristic:

una niña de diez años
= a girl of ten years (old)
(Her age is a property of the noun.)

a los diez años is a different structure and means “at age 10”, usually used for time references:

A los diez años empezó a jugar ajedrez.
= She started playing chess at the age of 10.

So:

de diez años → describes how old she is.
a los diez años → describes something that happened when she was that age.

Both are correct, but they’re used in different grammatical roles.