La bebé nació ayer.

Because the sentence is talking about a female baby. In Spanish, bebé can refer to either a male or a female baby, and the article shows the sex:

• el bebé = a baby boy / the male baby
• la bebé = a baby girl / the female baby

So here, la tells you the baby is female.

It is a common-gender noun: the noun form stays the same, and the article or adjective changes.

For example:

• el bebé pequeño
• la bebé pequeña

So bebé itself does not change to show masculine vs feminine in standard Spain Spanish.

Spanish does not normally say was born with a separate word like English does. Instead, it uses the verb nacer, which means to be born.

So:

• nació = was born

You do not usually say something like fue nacida for this meaning. That would sound unnatural in normal Spanish.

Nació comes from the infinitive nacer = to be born.

It is the third-person singular preterite form:

• yo nací
• tú naciste
• él / ella nació

So in La bebé nació ayer, the subject is la bebé, and the verb form matches she.

The preterite is used for a completed event in the past. A birth is seen as a single completed event, so the preterite is the normal tense here.

• nació = she was born

This is much more natural here than the present tense nace, which would mean is born / is being born in a different context.

Because with ayer, standard Spanish normally uses the preterite, not the present perfect.

In Spain Spanish, a useful contrast is:

• Ha nacido hoy = She was born today
• Nació ayer = She was born yesterday

So ayer strongly points to nació, not ha nacido.

The accent mark shows the stress: na-CIÓ.

It is also part of the normal spelling of the third-person singular preterite ending -ó:

• habló
• comió
• nació

Without the accent, nacio, the word would be spelled incorrectly and the stress would not be clear.

Because it is stressed on the last syllable: be-BÉ.

In Spanish, words ending in a vowel, n, or s are normally stressed on the second-to-last syllable unless there is a written accent. Since bebé is stressed on the last syllable, it needs the accent mark.

You can absolutely say Nació ayer if the context already makes it clear who you mean. Spanish often drops the subject when it is understood.

So both are possible:

• La bebé nació ayer = explicit subject
• Nació ayer = subject omitted because it is already clear

But saying just Bebé nació ayer without la would not sound natural in normal Spanish.

Yes. Spanish word order is more flexible than English.

These are all possible:

• La bebé nació ayer
• Ayer nació la bebé
• Ayer la bebé nació (possible, but less neutral)

The version you have, La bebé nació ayer, is a very natural, neutral order.

La bebé is correct and understandable in Spain. However, depending on context, Spaniards might also say:

• la niña if they simply mean the baby girl
• just nació ayer if the subject is already known

Also, la beba exists in some varieties of Spanish, but in Spain la bebé is the more standard and familiar choice.