A mi amiga le queda por decidir si quiere sentarse en la fila de delante o en la de atrás.

This is a very common Spanish pattern called clitic doubling.

• A mi amiga names the person explicitly.
• le is the indirect object pronoun that goes with the verb phrase.

Spanish often does this with indirect objects, especially when the person is mentioned at the start for topic or emphasis:

• A Juan le gusta el café.
• A mi hermana le queda poco tiempo.

So here, A mi amiga le queda... is completely normal. English usually would not repeat the person like this, but Spanish often does.

The structure quedar por + infinitive means to still remain to be done or to still have left to do.

So:

• Me queda por leer este libro = I still have this book left to read.
• Le queda por decidir... = She still has to decide...

It is a fairly natural way to talk about something pending. A more direct alternative would be todavía tiene que decidir.

It is singular because the sentence treats the pending matter as one thing: the decision about where she wants to sit.

In other words, what remains is a single issue, so Spanish uses singular queda.

Also, mi amiga is not the subject here. She is the person affected by the situation, shown by le.

Because por + infinitive in this pattern means still left to do or pending.

• queda por decidir = remains to be decided

By contrast, para + infinitive usually expresses purpose:

• Estudio para aprender = I study in order to learn

So queda para decidir would not work here.

Because this si means whether / if.

Spanish distinguishes:

• si = if / whether
• sí = yes

Here it introduces an indirect yes-no question:

• si quiere sentarse... = whether she wants to sit...

So it must be si, with no accent.

Because sentarse means to sit down / to take a seat.

Compare:

• sentarse = to sit down
• sentar = to seat someone, or sometimes to suit someone

Examples:

• Quiero sentarme. = I want to sit down.
• La profesora sentó a los alumnos. = The teacher seated the students.
• Ese color te sienta bien. = That colour suits you.

So in this sentence, since the friend is choosing where she will sit, the reflexive form sentarse is the right one.

With a conjugated verb plus an infinitive, Spanish often allows the pronoun in two places:

• quiere sentarse
• se quiere sentar

Both are possible here.

Attaching it to the infinitive is very common and often sounds especially natural. So quiere sentarse is just standard pronoun placement in a two-verb structure.

Because delante and atrás are normally adverbs, not adjectives. When Spanish wants to use them after a noun, it often uses de + adverb:

• la puerta de atrás
• la parte de delante
• la fila de delante

So de helps turn that idea into something that can modify the noun.

You could also hear adjectives like delantera or trasera, but de delante / de atrás is very natural in everyday speech.

This is ellipsis: Spanish leaves out a noun because it is already understood.

• en la de atrás really means en la fila de atrás

The article la stands in for la fila.

This is very common in Spanish:

• Prefiero la roja. = I prefer the red one.
• Nos vemos en la de siempre. = We’ll meet at the usual one/place.

So here, once fila has already appeared, Spanish does not need to repeat it.

No. A mi amiga is not the subject.

It is the person the situation relates to, shown again by the indirect object pronoun le. The idea that functions as the subject is the pending matter itself: the unresolved decision.

A useful way to think about it is:

• What remains?
  The decision about whether she wants to sit in the front row or the back row.

So mi amiga is the person involved, not the grammatical subject.

Not always.

• primera fila specifically means the first row
• fila de delante means the front row / the row at the front

In many situations they may refer to the same row, but de delante is a bit more relative and contrastive, especially when paired with de atrás. It is more like the front one versus the back one, rather than giving an exact numbered position.

Because si here introduces an indirect question, and in that kind of structure Spanish normally uses the indicative.

Compare:

• No sé si viene.
• Estoy pensando si voy.
• Tiene que decidir si quiere sentarse delante o atrás.

So quiere is the normal form here. The subjunctive quiera would not fit this sentence.