Learn Hardcore Spanish (Latin America): Voy a estudiar junto con mi hermana en la biblioteca. - I am going to study together with my sister at the library.

Voy a estudiar = ir a + infinitive (periphrastic future).
- Very common in everyday speech.
- Often used for near future or plans/intentions.
- Similar to English “I’m going to study.”
Estudiaré = simple future tense.
- Feels a bit more formal or distant/less immediate.
- Often used for predictions, promises, or a more abstract future.
- Closer to English “I will study.”

In Latin America, for a normal, concrete plan (especially soon), ir a + infinitive (voy a estudiar) sounds more natural in conversation.

Could I say Yo voy a estudiar... instead of Voy a estudiar...?

Yes, you can say Yo voy a estudiar junto con mi hermana en la biblioteca.

Spanish usually drops the subject pronoun (yo, tú, él...) because the verb ending already shows who the subject is.
You add yo mainly for emphasis, contrast, or clarity. For example:
- Yo voy a estudiar, pero ellos van a jugar.
  (I am going to study, but they are going to play.)

In your sentence, Voy a estudiar... is the default, neutral way. Yo voy a estudiar... emphasizes that you, specifically, are the one going to study.

Why is it voy a estudiar and not voy para estudiar?

Because ir a + infinitive is a fixed structure that expresses the future (like “going to do something” in English).

Ir a + infinitive = “to be going to [do something]”
- Voy a estudiar. – I’m going to study.
- Vamos a comer. – We’re going to eat.
Ir para + infinitive is not used for the future meaning. Para + infinitive is used to express purpose, but with a different structure:
- Voy a la biblioteca para estudiar. – I’m going to the library in order to study.

So:

Voy a estudiar = future plan (I’m going to study).
Voy a la biblioteca para estudiar = I’m going to the library with the purpose of studying.

They can sometimes describe similar situations, but they are grammatically different structures and para cannot replace a in ir a + infinitive.

What extra meaning does junto con mi hermana add compared to just con mi hermana?

Both are correct, but they feel slightly different.

Con mi hermana = with my sister (neutral).
Junto con mi hermana = roughly together with my sister, emphasizing being side by side / in company.

Junto con can:

Sound a bit more explicit about doing the action together, as a unit.
In some contexts, it can feel a bit more formal or emphatic.

In everyday conversation, many native speakers would simply say:

Voy a estudiar con mi hermana en la biblioteca.

Junto con is optional; it just highlights the idea of togetherness a bit more.

Can I say junto a mi hermana instead of junto con mi hermana?

You can, but it usually means something slightly different.

Junto con mi hermana
- Emphasizes doing something together, as companions:
  “together with my sister.”
Junto a mi hermana
- More about physical position: next to / beside my sister.
- Example:
  - Estoy sentado junto a mi hermana. – I’m sitting next to my sister.

In your sentence, Voy a estudiar junto con mi hermana focuses on studying together, not just physically sitting beside her, so junto con (or simply con) is more natural.

Can I change the word order, like Voy a estudiar en la biblioteca junto con mi hermana?

Yes, Spanish allows some flexibility in word order. These are all possible:

Voy a estudiar junto con mi hermana en la biblioteca.
Voy a estudiar en la biblioteca junto con mi hermana.
Junto con mi hermana, voy a estudiar en la biblioteca.
En la biblioteca voy a estudiar junto con mi hermana.

The basic meaning doesn’t change, but the focus can shift slightly:

Ending with en la biblioteca: a little more emphasis on where.
Ending with junto con mi hermana: a little more emphasis on with whom.
Starting with Junto con mi hermana: emphasizes the together with my sister part as an introduction.

All are grammatically correct and natural in the right context.

Why is it en la biblioteca and not just en biblioteca or en el biblioteca?

A few points:

Article + gender
- Biblioteca is a feminine noun, so it uses la, not el.
  - la biblioteca = the library
Needing the article
- In Spanish, you usually need the definite article with specific places:
  - en la biblioteca – in/at the library
- Saying en biblioteca is normally incorrect in this context (it sounds incomplete or like very technical/register-specific speech).
“En” vs “a”
- En la biblioteca = in/at the library (location).
- A la biblioteca = to the library (movement, direction).
  - Voy a la biblioteca. – I’m going to the library.
  - Voy a estudiar en la biblioteca. – I’m going to study in the library.

So en la biblioteca is the normal, correct way to say “in/at the library” here.

What is the difference between biblioteca and librería?

This is a classic false friend:

Biblioteca = library (place where you borrow books, usually for free).
Librería = bookstore / bookshop (place where you buy books).

So:

Voy a estudiar en la biblioteca. – I’m going to study in the library.
Voy a comprar un libro en la librería. – I’m going to buy a book at the bookstore.

Using librería instead of biblioteca would change the meaning.

Could I just say con mi hermana and drop mi, like in English “with sister”?

Normally, no. In Spanish you usually must use the possessive with family members:

con mi hermana – with my sister
con tu hermana – with your sister
con su hermana – with his/her/their sister

Just saying con hermana sounds incomplete or strange in standard Spanish, unless it’s a special context (for example, speaking about nuns: con hermana María, “with Sister María”).

So in normal family situations, keep the possessive: mi hermana.

How would I say “with my older/younger sister”?

You add mayor or menor after hermana:

con mi hermana mayor – with my older (or eldest) sister
con mi hermana menor – with my younger sister

So your full sentence could be:

Voy a estudiar junto con mi hermana mayor en la biblioteca.
Voy a estudiar junto con mi hermana menor en la biblioteca.

Why is voy the form of ir here, and what are its other main forms?

Voy is the first person singular (yo) form of the verb ir (to go) in the present tense.
Ir is highly irregular, so it doesn’t look like its infinitive.

Present tense forms of ir:

yo voy – I go / I’m going
tú vas – you go (informal singular)
él / ella / usted va – he / she goes; you go (formal singular)
nosotros / nosotras vamos – we go
vosotros / vosotras vais – you (plural, informal) [mostly Spain]
ellos / ellas / ustedes van – they go; you (plural) go

In voy a estudiar, voy means “I am going”, and the whole structure voy a + infinitive expresses a future plan.

Could I use the present tense Estudio junto con mi hermana... to talk about the future?

Sometimes, yes. Spanish can use the present to talk about the near future, similar to English:

Mañana estudio junto con mi hermana en la biblioteca.
Tomorrow I study / I’m studying together with my sister in the library.

Key points:

You usually need a time expression (like mañana, esta noche, el sábado) to make it clear it’s future.
Without a time word, Estudio junto con mi hermana en la biblioteca is normally understood as a habit:
- “I study together with my sister in the library (regularly).”

So for a clear statement of a plan, especially without extra context, Voy a estudiar... is the safest and most natural.