Si el pañal está sucio, mi hermana cambia a la bebé y le prepara un biberón.

Si means if.

Here it introduces a condition:

Si el pañal está sucio... = If the diaper is dirty...

This is a very common pattern in Spanish:

• Si tengo tiempo, voy. = If I have time, I go / I’ll go.
• Si llueve, nos quedamos en casa. = If it rains, we stay home.

In this sentence, both verbs are in the present because it describes a usual or repeated situation.

Spanish usually uses estar for a temporary state or condition, and ser for more permanent characteristics.

A diaper being dirty is a temporary condition, so:

• el pañal está sucio = the diaper is dirty

If you said es sucio, it would sound wrong here, because the diaper is not being described as inherently dirty by nature; it is just dirty at that moment.

Pañal is a masculine noun, so it takes:

• the masculine article el
• the masculine adjective form sucio

So:

• el pañal
• el pañal está sucio

If the noun were feminine, the adjective would change too:

• la camisa está sucia

This is basic gender agreement in Spanish: articles and adjectives must agree with the noun.

That a is the personal a.

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

• Veo a María.
• Ayudo a mi padre.
• Cambia a la bebé.

Even though in English we do not add anything before the baby, Spanish normally does.

So:

• cambia a la bebé = changes the baby

Yes, in context it can mean changes the baby, especially because the sentence has just mentioned the diaper.

That said, many speakers would use a more explicit version, such as:

• le cambia el pañal a la bebé = she changes the baby’s diaper
• cambia el pañal de la bebé = she changes the baby’s diaper

So cambia a la bebé is understandable, but it can be a little less explicit than the versions above. The context with pañal makes the meaning clear.

Bebé can be used as either masculine or feminine depending on the baby’s sex:

• el bebé = the baby boy / sometimes just the baby
• la bebé = the baby girl

In this sentence, la bebé tells you the baby is female.

This is normal even though the word ends in -é. Spanish noun gender is not always predictable from the ending.

Because le here is an indirect object pronoun, meaning to her.

The structure is:

• preparar algo a alguien = to prepare something for someone

So:

• un biberón = the thing being prepared → direct object
• a la bebé / for the baby = the recipient → indirect object

That is why Spanish uses le:

• le prepara un biberón = she prepares a bottle for her

Not la, because la is a direct object pronoun, and here the baby is not the direct object of preparar; the bottle is.

Because singular indirect object pronouns in Spanish do not change for gender.

So:

• le = to him / to her / to you (formal)

Examples:

• Le doy el libro. = I give him/her the book.
• Le preparo un biberón. = I prepare him/her a bottle.

So le is correct whether the baby is male or female.

Un biberón means a bottle, so it introduces it as one bottle, not a specific already-identified bottle.

• un = a/an
• el = the

So:

• le prepara un biberón = she prepares a bottle for her

If you said el biberón, it would suggest a specific bottle already known in the conversation:

• le prepara el biberón = she prepares the bottle

Both are possible in Spanish, but they mean slightly different things.

Because Spanish often leaves out subject pronouns when the verb form already makes the subject clear.

Here:

• cambia
• prepara

Both are third person singular, and the sentence already names the subject: mi hermana.

So Spanish does not need to repeat ella:

• mi hermana cambia... y le prepara...

Adding ella is possible, but usually unnecessary unless you want emphasis or contrast:

• Mi hermana cambia a la bebé y ella le prepara un biberón would sound more marked.

The present tense here expresses a habitual or general action.

The sentence means something like:

• whenever the diaper is dirty, my sister changes the baby and prepares her a bottle

Spanish often uses the present this way for routines and repeated situations:

• Si hace frío, me pongo un abrigo.
• Si el niño llora, su madre lo calma.

So the present does not only mean right now; it can also describe what usually happens.