A veces oigo a mi hermana tararear en la cocina mientras prepara el desayuno.

A veces means sometimes. It is a very common time expression, and it often goes at the beginning of the sentence to set the scene:

A veces oigo a mi hermana tararear...

You could also place it elsewhere, such as Oigo a mi hermana tararear a veces, but starting with A veces is very natural.

Both oír and escuchar relate to hearing, but they are not exactly the same.

• oír = to hear
• escuchar = to listen

In this sentence, oigo is used because the idea is that the speaker hears the humming, not necessarily that they are actively paying attention to it.

So:

• Oigo a mi hermana tararear = I hear my sister humming
• Escucho a mi hermana tararear = I listen to my sister humming

Both are possible, but oír fits the meaning of simply perceiving the sound.

Yes, oír is irregular.

Its present tense forms are:

• yo oigo
• tú oyes
• él/ella/usted oye
• nosotros oímos
• vosotros oís
• ellos/ellas/ustedes oyen

So oigo is the correct yo form.

This is the personal a.

In Spanish, when the direct object is a specific person, you usually put a before it.

So:

• Oigo a mi hermana = I hear my sister

Compare:

• Oigo música = I hear music
• Oigo a mi hermana = I hear my sister

The a does not mean to here. It is just a marker used with people.

After verbs of perception like ver, oír, and sentir, Spanish very often uses this structure:

verb of perception + object + infinitive

So:

• Oigo a mi hermana tararear
• literally: I hear my sister hum

This is very natural in Spanish.

You can sometimes use a clause with que, but in this kind of sentence the infinitive structure is much more common and more elegant.

Because it depends on the verb of perception oigo.

The pattern is:

oír + a + person + infinitive

Examples:

• Oigo a Juan cantar = I hear Juan sing/singing
• Veo a los niños jugar = I see the children play/playing
• Siento al bebé respirar = I feel the baby breathe/breathing

So tararear stays in the infinitive because it is part of that construction.

Tararear means to hum, especially humming a tune without singing the actual words.

It is a very specific verb and quite useful.

Examples:

• Tarareo cuando estoy contento. = I hum when I’m happy.
• Está tarareando una canción. = She’s humming a song.

The subject is omitted because Spanish often leaves out subject pronouns when they are understood from context.

In mientras prepara el desayuno, the implied subject is mi hermana.

So the meaning is:

• while she prepares breakfast

Spanish does this all the time:

• María canta mientras cocina. = María sings while she cooks.

There is no need to repeat ella unless you want emphasis or contrast.

In normal interpretation, it refers to mi hermana.

That is because the most natural subject to continue with after oigo a mi hermana tararear is the sister herself. So the listener understands:

• I hear my sister humming in the kitchen while she prepares breakfast.

If you wanted to make it clearly refer to the speaker, you would normally say something more explicit, such as:

• ...mientras yo preparo el desayuno.

Spanish often uses the simple present where English might use the present continuous.

So:

• mientras prepara el desayuno
  can mean
• while she prepares breakfast
  or
• while she is preparing breakfast

Both are possible in English, and the Spanish simple present is perfectly natural here.

If you said mientras está preparando el desayuno, it would also be correct, but slightly more explicitly focused on the ongoing action.

It most naturally describes where the sister is when this is happening: in the kitchen.

In practice, it fits the whole scene:

• you hear your sister
• humming
• in the kitchen
• while she prepares breakfast

Spanish often places location phrases like en la cocina after the infinitive or action phrase. This sounds very natural.

Yes, Spanish word order is flexible, though some versions sound more natural than others.

For example:

• A veces oigo a mi hermana tararear en la cocina mientras prepara el desayuno.
• Oigo a mi hermana tararear en la cocina a veces mientras prepara el desayuno.
• A veces, mientras prepara el desayuno, oigo a mi hermana tararear en la cocina.

These all work, but the original sentence is smooth and natural.

Yes, preparar el desayuno is the normal way to say to prepare breakfast.

In Spanish, meals often take the definite article:

• el desayuno = breakfast
• la comida = lunch / the midday meal
• la cena = dinner

So:

• prepara el desayuno = she prepares breakfast

That is more natural than leaving out the article here.

In Spain, a broad pronunciation guide would be:

A veces oigo a mi hermana tararear en la cocina mientras prepara el desayuno

Some useful pronunciation notes:

• c in veces and cocina is pronounced like th in much of Spain: ve-thes, ko-thi-na
• oi in oigo is a diphthong, roughly OY-go
• h in hermana is silent
• rr in tararear is a rolled or tapped r, depending on the exact position and speaker
• z in desayuno is also pronounced like th in most of Spain

Not directly in the same structure if you want to keep it natural.

With a noun, you say:

• Oigo a mi hermana tararear

If you use a pronoun, Spanish often prefers an object pronoun before the verb:

• La oigo tararear = I hear her humming

Here la refers to mi hermana.

So yes, it can be replaced, but the structure changes a bit.

A very useful pattern is:

A veces + verb of perception + personal a + person + infinitive + place/time clause

In this sentence:

• A veces = sometimes
• oigo = I hear
• a mi hermana = my sister
• tararear = humming / hum
• en la cocina = in the kitchen
• mientras prepara el desayuno = while she prepares breakfast

This pattern is extremely common in Spanish, especially with:

• ver = to see
• oír = to hear
• escuchar = to listen to
• sentir = to feel / hear / sense

So this sentence is a great model to learn from.