Learn Hardcore Spanish (Spain): Mi hermana trae un sándwich y una pelota al parque. - My sister brings a sandwich and a ball to the park.

mi is used with singular nouns: mi hermana (my sister), mi hermano (my brother)
mis is used with plural nouns: mis hermanas (my sisters), mis hermanos (my brothers)

Mi does not change for gender, only for number. So:

masculine singular: mi hermano
feminine singular: mi hermana
masculine plural: mis hermanos
feminine plural: mis hermanas

How is trae formed, and which subject does it go with?

Trae is the 3rd person singular of the verb traer (to bring) in the present indicative.

Present indicative of traer:

yo traigo
tú traes
él / ella / usted trae
nosotros / nosotras traemos
vosotros / vosotras traéis (Spain)
ellos / ellas / ustedes traen

In the sentence Mi hermana trae…, the subject is mi hermana (3rd person singular), so the verb must be trae.

What’s the difference between traer and llevar here? Could I say Mi hermana lleva…?

Standard distinction:

traer = to bring towards the place where the speaker is (or will be)
llevar = to take away from the speaker’s place to another place

So:

Mi hermana trae un sándwich y una pelota al parque.
Suggests the speaker is (or will be) at the park; she is bringing them to me / where I am.
Mi hermana lleva un sándwich y una pelota al parque.
Focuses on the fact that she is taking them there, without implying that the speaker is in the park.

In practice, many native speakers sometimes mix them, but if you want to be precise in Spain, use:

traer when something comes to where you are
llevar when something goes to somewhere else

Why does Spanish use the simple present trae instead of something like “is bringing”?

Spanish simple present covers two English uses:

general / habitual: Mi hermana trae un sándwich al parque todos los días. = My sister brings a sandwich to the park every day.
current / near future action: Mi hermana trae un sándwich al parque (ahora / luego). = My sister is bringing a sandwich to the park (now / later).

You can use the progressive (está trayendo) to highlight an action in progress right now:

Mi hermana está trayendo un sándwich al parque. = My sister is (right now) bringing a sandwich to the park.

But the simple trae is very common and often enough, especially if context shows whether it’s habitual or happening now/soon.

Why is it un sándwich but una pelota? How do gender and articles work here?

In Spanish, every noun has grammatical gender:

sándwich is masculine → un sándwich
pelota is feminine → una pelota

The indefinite articles agree with the gender and number of the noun:

masculine singular: un sándwich
feminine singular: una pelota
masculine plural: unos sándwiches
feminine plural: unas pelotas

Note on sándwich:

It has an accent: sán‑dwich, stressed on the first syllable.
Plural in Spain is usually sándwiches.

In Spain, is sándwich the usual word, or would people say bocadillo?

Both exist, but they’re not exactly the same in Spain:

sándwich: usually a sandwich made with sliced loaf bread (pan de molde), often square, like many English‑style sandwiches.
bocadillo: a sandwich made with Spanish bread, typically a baguette-like roll.

So:

un sándwich de jamón y queso = ham and cheese on sliced bread
un bocadillo de jamón = ham in a baguette-style roll

In your sentence, un sándwich is fine if you specifically imagine sliced-bread sandwich.

What’s the nuance between pelota, balón, and bola?

In Spain:

pelota
- general word for a ball, often smaller / lighter balls
- used in many sports: pelota de tenis, pelota de béisbol, pelota de playa
balón
- tends to refer to larger, usually inflatable sports balls
- balón de fútbol, balón de baloncesto, balón de balonmano
bola
- more general “ball / sphere”: bola de nieve, bola de cristal, bola de billar
- also used colloquially in other senses (e.g. lie, lump).

In everyday speech, many people just say pelota as the generic word for “ball,” so una pelota fits fine here.

Why is it al parque and not a el parque? And what’s the difference between a, en, and para with places?

Al is a mandatory contraction of a + el:

a + el → al
- Voy al parque. (not a el parque)
a + la / los / las do not contract:
- Voy a la playa.
- Voy a los museos.

In your sentence, al parque = a + el parque → al parque.

Prepositions with places:

a: direction / destination – to
- Voy al parque. = I’m going to the park.
en: location – in / at
- Estoy en el parque. = I’m in/at the park.
para: purpose / intended destination – for / in order to
- Llevo la pelota para el parque. (for use in the park / intended for the park)

So here a is correct because the sister is bringing things to the park.

Why isn’t there a personal a before un sándwich y una pelota?

Spanish uses the personal a before direct objects that are:

specific people
or personified animals / things

Examples:

Veo a mi hermana.
Escucho a los niños.

But it is not used with inanimate things:

Veo el coche. (no a)
Tengo un libro.

In your sentence, un sándwich y una pelota are direct objects but they are things, not people, so:

Mi hermana trae un sándwich y una pelota al parque.
(no a before un sándwich y una pelota).

How would the sentence change in the plural, or if I replace the objects with pronouns?

Plural subject and objects

Singular: Mi hermana trae un sándwich y una pelota al parque.
Plural: Mis hermanas traen sándwiches y pelotas al parque.

Changes:

Mi hermana → Mis hermanas (plural possessive + plural noun)
trae → traen (3rd person singular → plural)
un sándwich → sándwiches (plural)
una pelota → pelotas (plural)

Replacing un sándwich y una pelota with a direct object pronoun

Together, un sándwich (masc.) and una pelota (fem.) become masculine plural for pronouns:

Mi hermana los trae al parque. = My sister brings them to the park.

Notes:

los refers to both items together (default to masculine plural).
The direct object pronoun goes before the conjugated verb: los trae.