Si el viento sigue así, el cucurucho se me va a llenar de arena antes de que termine de comerlo.

Because this is a real, possible condition, so Spanish normally uses the present indicative after si:

• Si el viento sigue así... = If the wind keeps up like this...

In Spanish, si is not usually followed by the present subjunctive for normal if clauses.
Compare:

• Si viene, salimos. = If he comes, we’ll go out.
• Si hace sol, vamos a la playa. = If it’s sunny, we’re going to the beach.

You do get other tenses after si, but not the present subjunctive in this kind of sentence.

Seguir means to continue / to keep on, and así means like this / this way.

So:

• el viento sigue así = the wind keeps being like this
• more naturally: if the wind keeps up like this

It is a very common Spanish way to talk about a situation continuing in the same manner:

• Si sigues así, te vas a poner malo. = If you keep this up, you’re going to get sick.
• Como siga así, suspende. = If he keeps going like this, he’ll fail.

In Spain, cucurucho often means a cone-shaped holder, especially an ice-cream cone. Depending on context, it can refer to:

• the edible cone itself
• the cone with the ice cream in it
• sometimes any cone-shaped paper holder

In this sentence, it most naturally suggests an ice-cream cone at the beach. That is why comerlo makes sense: it is something the speaker is eating.

This is one of the most useful things to notice in the sentence.

1. llenarse

Spanish often uses the pronominal form llenarse for to become full / to get filled up:

• El vaso se llenó de agua. = The glass filled up with water.
• El cucurucho se va a llenar de arena. = The cone is going to fill with sand.

2. The extra me

The me is an indirect object pronoun showing who is affected:

• se me va a llenar = it’s going to fill up on me / it’s going to get full for me

This adds the idea that it happens to my annoyance or inconvenience. English often does not translate this directly, but the feeling is something like:

• my cone is going to end up full of sand
• the cone’s going to get full of sand on me

So se me gives the sentence a more natural, personal, slightly frustrating tone.

Here it is best understood as a pronominal/inchoative use of llenarse plus an indirect object pronoun (me).

It is not really a passive.
It is also not a true reflexive in the sense of the cone fills itself on purpose.

Instead, it means the cone becomes full:

• El cubo se llenó de agua. = The bucket filled up with water.
• La playa se llenó de gente. = The beach filled up with people.

Then me adds the idea that this affects me personally:

• Se me rompió el móvil. = My phone broke on me.
• Se me ha mojado la toalla. = My towel got wet on me.
• Se me va a llenar de arena. = It’s going to get full of sand on me.

This is a very common Spanish pattern.

Yes. Both are correct:

• se va a llenar
• se llenará

But there is a slight difference in feel.

Ir a + infinitive

• se va a llenar
• often feels more immediate, conversational, and tied to what is clearly about to happen

Simple future

• se llenará
• can sound a bit more neutral, formal, or predictive

In everyday speech, especially in Spain, ir a + infinitive is very common:

• Va a llover.
• Te vas a caer.
• Se me va a llenar de arena.

With llenar, Spanish normally uses de for the substance or thing that fills something:

• llenar el vaso de agua = fill the glass with water
• llenar la habitación de humo = fill the room with smoke
• llenarse de arena = get full of sand

So de is the expected preposition here.

Con can appear in other contexts, but with llenar, de is the standard choice.

Because antes de que is followed by the subjunctive when the action has not yet happened and is being viewed as anticipated or pending.

So:

• antes de que termine = before I finish

This is one of the standard triggers for the subjunctive:

• Te llamo antes de que salgas. = I’ll call you before you leave.
• Hazlo antes de que sea tarde. = Do it before it’s too late.

If there is no change of subject and you use an infinitive, then it is antes de + infinitive:

• antes de salir = before leaving
• antes de comerlo = before eating it

But here the sentence is built with antes de que + finite verb, so it needs the subjunctive: termine.

In Spanish, terminar de + infinitive means to finish doing something:

• terminar de leer = to finish reading
• terminar de comer = to finish eating
• terminar de hacerlo = to finish doing it

So:

• antes de que termine de comerlo = before I finish eating it

The de belongs to the verb pattern terminar de + infinitive.

Be careful not to confuse it with another common use:

• acabar de + infinitive = to have just done something

Example:

• Acabo de comer. = I’ve just eaten.

But here terminar de comer means finish eating, not have just eaten.

It refers back to el cucurucho.

That may feel slightly odd at first if you are imagining only the cone itself, but in context cucurucho can mean the cone as the thing being eaten, often basically the whole ice-cream cone.

So:

• el cucurucho → masculine singular
• therefore the direct object pronoun is lo

That gives:

• comerlo = eat it

Because with an infinitive, object pronouns are normally attached to the end:

• comerlo = to eat it
• hacerlo = to do it
• verla = to see her/it

In this sentence, the infinitive is comer, so the pronoun joins it:

• termine de comerlo

If the verb were conjugated, the pronoun would normally go before it:

• No lo como. = I’m not eating it.
• Lo terminé. = I finished it.

With verb combinations, Spanish often allows two positions, but here the pronoun naturally goes with the infinitive comer.

Yes, that is possible, and it is a useful comparison.

With antes de que + subjunctive

• antes de que termine de comerlo

With antes de + infinitive

• antes de terminar de comerlo

Both can work here because the subject of terminar is the same speaker.

The version with antes de que termine is perfectly natural and slightly more explicit as a full clause.
The infinitive version is a bit more compact.

Both mean essentially the same thing in this context.

Yes. The combination se me is extremely common in Spanish for events that happen to someone, often unintentionally or inconveniently.

Examples:

• Se me cayó el helado. = I dropped my ice cream. / My ice cream fell on me.
• Se me perdió la llave. = I lost the key. / The key got lost on me.
• Se me va a llenar de arena el cucurucho. = My cone is going to get full of sand on me.

This structure often sounds less like a deliberate action and more like something that happens and affects the speaker.

That is why the sentence feels very natural: the speaker is complaining about an annoying situation, not describing a deliberate act.

The given order is natural, but Spanish allows some flexibility.

Original:

• Si el viento sigue así, el cucurucho se me va a llenar de arena antes de que termine de comerlo.

Other possible versions:

• Si el viento sigue así, se me va a llenar de arena el cucurucho antes de que termine de comerlo.
• El cucurucho se me va a llenar de arena, si el viento sigue así, antes de que termine de comerlo.

The original order is probably the clearest and most neutral.

Putting el cucurucho later can slightly emphasize the whole event first:

• se me va a llenar de arena el cucurucho

That kind of variation is common in spoken Spanish.

Spanish uses definite articles more often than English does.

• el viento = the wind
• el cucurucho = the cone

In English, you might sometimes say if wind keeps up like this only in a very reduced style, but standard English usually also says the wind.
In Spanish, the article is very natural here.

Also, when talking about a specific object already present in the situation, Spanish commonly uses the definite article:

• Pásame la toalla. = Pass me the towel.
• El helado se derrite. = The ice cream is melting.
• El cucurucho se me va a llenar de arena.

So there is nothing unusual about the articles in this sentence.