Mi hija sopló las velas tan rápido que una casi cayó sobre la tarta.

Because mi without an accent is the possessive adjective meaning my.

• mi hija = my daughter

mí with an accent is a pronoun used after prepositions:

• para mí = for me
• a mí = to me

So in this sentence, mi hija is correct because it shows possession.

Sopló is the preterite form of soplar (to blow), for él / ella / usted.

• mi hija sopló = my daughter blew

The preterite is used because this is a completed action in the past: she blew out the candles at a specific moment.

Conjugation of soplar in the preterite:

• yo soplé
• tú soplaste
• él/ella/usted sopló
• nosotros soplamos
• vosotros soplasteis
• ellos/ellas/ustedes soplaron

Las velas means the candles.

• vela = candle
• velas = candles
• las = the (feminine plural)

Spanish nouns have gender, and vela is feminine, so the article must match:

• la vela
• las velas

Yes, literally it is blew the candles, but in Spanish soplar las velas naturally means to blow out the candles in a birthday context.

Spanish often does not need a separate word for out here. The meaning is understood from the situation.

You could compare:

• soplar las velas = blow out the candles

This is a very natural expression.

This is a very common Spanish structure meaning so ... that.

• tan = so
• rápido = quickly / fast
• que = that

So:

• tan rápido que = so quickly that

Examples:

• Habló tan bajo que no le oí. = He spoke so quietly that I didn’t hear him.
• Corrió tan rápido que ganó. = He ran so fast that he won.

Because in Spanish, many adjectives can also be used adverbially, especially in everyday language.

So both of these can work:

• sopló rápido
• sopló rápidamente

Both mean she blew quickly, but rápido often sounds more natural and conversational.

This is very common in Spanish:

• habla claro = he speaks clearly
• camina lento = he walks slowly
• ve rápido = go quickly

Because una here stands for one candle.

After mentioning las velas, Spanish can avoid repeating the noun:

• una = one / one of them

So:

• una casi cayó = one almost fell

This is similar to English, where we might say:

• The candles were crooked and one almost fell.

Since vela is feminine, Spanish uses una, not uno.

Cayó is the preterite form of caer (to fall) for él / ella / usted.

Here it refers to una = one candle.

• una casi cayó = one almost fell

The preterite is used because this is again a single completed event in the past.

Conjugation of caer in the preterite:

• yo caí
• tú caíste
• él/ella/usted cayó
• nosotros caímos
• vosotros caísteis
• ellos/ellas/ustedes cayeron

Casi means almost, and it normally goes before the word or phrase it modifies.

• casi cayó = almost fell

That is the natural word order in Spanish.

Other examples:

• casi llego tarde = I almost arrive late / I was almost late
• casi me caigo = I almost fall / I almost fell

Sobre means onto / on top of / over, and it fits well here because the candle nearly fell onto the cake.

• sobre la tarta = onto the cake / on the cake

En la tarta would usually sound more like in the cake or simply less natural in this situation.

So if a candle tips and lands on the top surface, sobre la tarta is a good choice.

Yes, tarta is especially common in Spain for cake, particularly a celebration cake.

In many parts of Latin America, people often say pastel instead.

So:

• Spain: tarta de cumpleaños
• many Latin American countries: pastel de cumpleaños

Both are correct Spanish, but tarta sounds very natural for Spain.

Because caer means simply to fall, and that is enough here.

• cayó = it fell

Caerse can also be used, and often it suggests the fall happened accidentally or that something fell over/down. In many contexts, both are possible, but cayó is completely natural here.

Compare:

• La vela cayó sobre la tarta. = The candle fell onto the cake.
• La vela se cayó. = The candle fell down / toppled over.

In this sentence, cayó is straightforward and idiomatic.

It breaks down like this:

• Mi hija = subject
• sopló = main verb
• las velas = direct object
• tan rápido que... = result clause, meaning so quickly that...
• una = subject of the second clause, meaning one candle
• casi cayó = verb phrase
• sobre la tarta = prepositional phrase showing where it almost fell

So the pattern is roughly:

[subject] + [past action] + [object] + tan + [adverb] + que + [result]

This is a very useful pattern to learn.

It would change the feel of the sentence.

The original:

• Mi hija sopló las velas tan rápido que una casi cayó sobre la tarta.

This tells us about a specific completed moment.

If you used the imperfect, for example:

• Mi hija soplaba las velas...

it would sound like an ongoing or habitual action, which is less natural here unless you are describing the scene in progress.

For a one-time birthday moment, the preterite is the right choice:

• sopló
• cayó

Yes. Here tan rápido modifies the verb sopló, so it describes how she blew.

It does not mean the candles were fast. It means she blew so quickly.

That is why rápido functions adverbially here:

• sopló rápido = she blew quickly
• sopló tan rápido que... = she blew so quickly that...

You might be understood, but it sounds less natural.

The most natural order is:

• Mi hija sopló las velas tan rápido que...

That keeps las velas close to sopló and then adds the description of how she did it.

Spanish word order is flexible, but not every possible order sounds equally natural. In this case, the original version is the best choice.