Learn Hardcore Spanish (Latin America): Hoy voy a la biblioteca con mi hermana. - Today I go to the library with my sister.

voy = I go / I am going
Because voy only works with yo (I), saying yo is usually optional:
Voy a la biblioteca. = Yo voy a la biblioteca.

You add yo mainly:

for emphasis: Yo voy a la biblioteca, no tú. (I am going, not you.)
to contrast subjects: Yo voy, pero él se queda. (I’m going, but he’s staying.)

In everyday speech, Voy a la biblioteca is the most natural.

What verb is voy from, and how is it conjugated?

Voy is the first-person singular (I) of the verb ir (to go), which is irregular.

Present tense of ir:

yo voy – I go / I am going
tú vas – you go (informal, singular)
usted va – you go (formal, singular)
él / ella va – he / she goes
nosotros / nosotras vamos – we go
ustedes van – you all go (plural, Latin America)
ellos / ellas van – they go

So in the sentence:
Hoy voy a la biblioteca = Today I’m going to the library.

Why is it a la biblioteca and not en la biblioteca or para la biblioteca?

The preposition a is used for going to a place (destination or direction):

Voy a la biblioteca. – I’m going to the library.

Comparison:

en la biblioteca = in/at the library (location, not movement)
- Estoy en la biblioteca. – I am in/at the library.
para la biblioteca = for the library (purpose or recipient)
- Compro libros para la biblioteca. – I’m buying books for the library.

So with verbs of movement like ir, use a for “to (a place)”.

Why is it la biblioteca and not una biblioteca?

La is the definite article (the), while una is the indefinite article (a / an).

Voy a la biblioteca. – I’m going to the library.
→ Usually means a specific, known library (the one you usually go to, or the one both speakers know about).
Voy a una biblioteca. – I’m going to a library.
→ Any library, not a specific or previously known one.

In everyday speech, people often say la biblioteca when talking about their usual or local library, even if they haven’t just mentioned it.

Can I say Voy a la biblioteca hoy instead of Hoy voy a la biblioteca? Is the word order important?

Both are correct:

Hoy voy a la biblioteca.
Voy a la biblioteca hoy.

The meaning is the same: I’m going to the library today.

Differences:

Putting hoy at the beginning (Hoy voy…) slightly emphasizes today.
Putting hoy at the end (… biblioteca hoy) is also common and sounds natural.

In Spanish, time expressions like hoy, mañana, esta tarde move quite freely: start, middle, or end of the sentence are all possible.

What’s the difference between mi and mí? Why is it mi hermana without an accent?

They are two different words:

mi (no accent) = my (possessive adjective)
- mi hermana – my sister
- mi casa – my house
mí (with accent) = me (stressed object pronoun, used after prepositions)
- para mí – for me
- a mí – to me

In the sentence, you’re saying my sister, so you must use the possessive adjective mi:
…con mi hermana. – with my sister.

Why is it hermana and not hermano here?

Spanish nouns have grammatical gender:

hermano = brother (masculine)
hermana = sister (feminine)

Because the person is female (sister), you use hermana.

Some related forms:

mis hermanos – my brothers / my siblings (can mean mixed group)
mis hermanas – my sisters (all female)

How would I say with my sisters instead of with my sister?

You make both the article/possessive and the noun plural:

con mi hermana – with my sister (one)
con mis hermanas – with my sisters (more than one)

So the full sentence becomes:
Hoy voy a la biblioteca con mis hermanas. – Today I’m going to the library with my sisters.

Is con always “with”? Could I leave it out?

Con is the standard preposition for with:

con mi hermana – with my sister

You cannot just drop con in this sentence;
Hoy voy a la biblioteca mi hermana is incorrect.

However, you can rephrase the idea without con, but then you change the structure:

Hoy voy a la biblioteca y mi hermana también.
Today I’m going to the library and my sister is too.

So: for the simple with X meaning after a verb of movement, you need con.

How do you pronounce hoy voy a la biblioteca? Any tricky sounds?

Key points:

hoy: the h is silent. Sounds like oy in boy but shorter: [oi].
voy: the v is pronounced like a soft b in most Spanish dialects: [boj] (boi).
a la: both a are like the a in father, not like English day.
biblioteca:
- bi-blio-te-ca (4 syllables)
- Stress on te: bib-lio-TE-ca
- Both b sounds are like English b, but in fast speech the second can be softer.

Altogether (rough guide, Latin American):
[oi boj a la bib-lio-TE-ka]

Does hoy voy a la biblioteca mean “I go” or “I am going”? How do I say “I’m going to go”?

In Spanish, the simple present often covers both:

Hoy voy a la biblioteca.
= Today I go to the library.
= Today I am going to the library.

To say I’m going to go, you use ir a + infinitive:

Hoy voy a ir a la biblioteca. – Today I’m going to go to the library.

Native speakers often prefer the shorter Hoy voy a la biblioteca when the context is clearly future/today, but voy a ir is also very natural and emphasizes the plan or intention.

Is hoy always “today”? Are there similar words I should know?

Yes, hoy means today.

Some related time words:

ayer – yesterday
mañana – tomorrow
hoy en la mañana / esta mañana – this morning
hoy en la tarde / esta tarde – this afternoon
hoy en la noche / esta noche – tonight

The structure stays the same:

Mañana voy a la biblioteca con mi hermana. – Tomorrow I’m going to the library with my sister.
Ayer fui a la biblioteca con mi hermana. – Yesterday I went to the library with my sister.

How would the sentence change if it were someone else going, like “Today my sister is going to the library”?

You change the subject and the verb form:

Original: Hoy voy a la biblioteca con mi hermana.
Today I’m going to the library with my sister.
Today my sister is going to the library.
→ Hoy mi hermana va a la biblioteca.

Changes:

Subject: mi hermana (my sister)
Verb form: va (third person singular of ir) instead of voy
You no longer need con mi hermana, because she is the subject.