Learn Hardcore Spanish (Latin America): Ella se unió a nuestro club porque quiere practicar con hablantes nativos y comparar su idioma materno con el nuestro. - She joined our club because she wants to practice with native speakers and compare her mother tongue with ours.

If you said Ella unió nuestro club, it would mean She joined our club (to something else), which is wrong in this context. The se marks that she herself is becoming part of the group.

Why is it unió (past tense) instead of se une or se ha unido?

Unió is the preterite (simple past), and it presents the action as a completed event at a specific point in time.

Ella se unió a nuestro club = She joined our club (at some point in the past; the joining is done).

Other possibilities and their nuances:

Ella se une a nuestro club – present tense: She joins / is joining our club (right now, or as a general fact).
Ella se ha unido a nuestro club – present perfect: She has joined our club (emphasizes the result now; more common in Spain than in much of Latin America, where the simple past often covers this meaning).

For Latin American Spanish, se unió is very natural when narrating a past event.

Why do we say unirse a nuestro club and not unirse en nuestro club or something else?

With the verb unirse in the sense of joining a group, Spanish uses the preposition a:

unirse a un club
unirse a un grupo
unirse a la conversación

So a nuestro club is required by the verb unirse.

En nuestro club would mean in our club (a location), not join our club. For example:

Ella practica español en nuestro club. = She practices Spanish in our club.

Why is porque written as one word here and not por qué?

Porque (one word) is a conjunction meaning because and introduces a reason:

Ella se unió a nuestro club porque quiere practicar…
She joined our club because she wants to practice…

Por qué (two words) is used mainly in questions and means why:

¿Por qué se unió ella a nuestro club?
Why did she join our club?

So:

porque = because
por qué = why

Why is it quiere practicar and not something like quiere que practica?

After querer (to want) in Spanish:

If the same person performs the second action, you use the infinitive:
- Ella quiere practicar. = She wants to practice.
If a different person performs the second action, you use querer que + subjunctive:
- Ella quiere que nosotros practiquemos. = She wants us to practice.

In the sentence, she is the one who wants to practice, so we use the infinitive practicar:

…porque quiere practicar con hablantes nativos…

Why is the tense quiere (present) and not quería or quiso?

Quiere is present tense and expresses a current, ongoing desire:

Ella se unió… porque quiere practicar…
She joined… because she wants (now) to practice…

Alternatives would slightly change the meaning:

quería practicar – she wanted to practice (back then; past background desire).
quiso practicar – she tried or decided to practice (often punctual, completed).

Using quiere connects the past action (se unió) with her current motivation; she joined, and she still wants to practice.

Why is it con hablantes nativos and not just con nativos?

Both are possible, but not identical in nuance:

hablantes nativos = native speakers (explicitly about language)
nativos (alone) = natives, which could mean native people of a place (not necessarily focused on language).

In a language-learning context, hablantes nativos makes it clear we’re talking about people who are native speakers of the language.

Also note:

hablantes = speakers (a noun)
nativos = native (used here as an adjective modifying hablantes)

So hablantes nativos literally = native speakers.

Can hablantes nativos refer to both men and women, or do I need a feminine form?

Hablantes nativos is gender-neutral in practice when you don’t specify gender:

hablantes ends in -es and doesn’t show gender.
nativos is grammatically masculine plural, and Spanish masculine plural is the default when the group is mixed or gender is unspecified.

If you really want to be explicitly inclusive, you might see:

hablantes nativos y nativas
hablantes nativos/as (informal writing)

But hablantes nativos is the normal, standard phrase for native speakers in general.

Why is it idioma materno and not idioma materna?

Adjectives in Spanish must agree in gender and number with the noun they describe.

idioma is masculine singular.
So the adjective must also be masculine singular: materno.

Hence:

idioma materno (correct: masculine).
lengua materna (also correct, but now lengua is feminine, so the adjective changes to materna).

Both idioma materno and lengua materna mean mother tongue / native language.

Who does su in su idioma materno refer to? Could it be ambiguous?

Su can mean his / her / their / your (formal), so yes, in isolation it can be ambiguous.

In this specific sentence:

Ella se unió… porque quiere practicar… y comparar su idioma materno con el nuestro.

The most natural interpretation is that su refers to ella (her), so:

su idioma materno = her mother tongue.

To avoid ambiguity, Spanish sometimes uses more explicit phrases:

el idioma materno de ella = her mother tongue
su propio idioma materno = her own mother tongue (emphasizes it’s hers)

But usually context makes su clear enough.

Why does the sentence say con el nuestro instead of just con nuestro?

Here, el nuestro is a possessive pronoun, meaning ours:

el nuestro = our one / our (language)

Spanish possessive pronouns normally need the definite article:

el mío, la mía, los míos, las mías
el nuestro, la nuestra, los nuestros, las nuestras, etc.

If you said con nuestro idioma, nuestro would be a possessive adjective, and you’d need the noun:

con nuestro idioma = with our language

So:

comparar su idioma materno con el nuestro
= compare her mother tongue with ours.
comparar su idioma materno con nuestro idioma
= compare her mother tongue with our language.

Both are correct; the original just omits the repeated noun and uses the pronoun.

Could the sentence also be comparar su idioma materno y el nuestro without con?

You can say:

comparar su idioma materno y el nuestro

but it sounds slightly less natural and more like you’re just listing the two languages rather than explicitly setting up a comparison.

For comparisons, comparar X con Y is the standard, clear pattern:

comparar el español con el inglés
comparar su idioma materno con el nuestro

So the original comparar su idioma materno con el nuestro is the most idiomatic for compare X with Y.

Is it necessary to say Ella, or could I just say Se unió a nuestro club…?

In Spanish, the subject pronoun is often optional because the verb ending already shows the person:

(Ella) se unió a nuestro club…

Both are grammatically correct. The difference:

Without Ella: more neutral, typical Spanish style.
With Ella: adds a bit of emphasis or clarity, especially if you’re contrasting her with someone else or introducing her as a topic.

For example:

Ella se unió a nuestro club, pero su hermano no quiso.
She joined our club, but her brother didn’t want to.