Hoy voy a la biblioteca con mi hermana.

In Spanish, the verb ending already shows who the subject is.

• 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.

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.

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)”.

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.

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.

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.

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)

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.

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.

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]

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.

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.

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.