Si el helado se derrite antes de llegar a la playa, prefiero comprar fruta en lugar de otro postre.

Because derretirse is the usual intransitive form meaning to melt or to melt by itself.

• Derretir = to melt something
  
  • El sol derrite el helado = The sun melts the ice cream.
• Derretirse = to melt
  
  • El helado se derrite = The ice cream melts.

Here, the ice cream is not melting something else; it is the thing undergoing the change, so se derrite is the natural choice. The se here is not really reflexive in the English sense; it is part of the verb form.

Because this is a real or possible condition, not a hypothetical one.

A very common pattern in Spanish is:

si + present indicative, + present/future/imperative

So:

• Si el helado se derrite..., prefiero...
  = If the ice cream melts..., I prefer...

This sounds like a normal, realistic situation.

If it were more hypothetical, Spanish would change the structure:

• Si el helado se derritiera..., preferiría...
  = If the ice cream were to melt..., I would prefer...

Because after si in ordinary conditional sentences, Spanish normally uses the indicative, not the subjunctive.

So you say:

• Si se derrite...
  not
• Si se derrita...

That is one of the first big rules learners need to remember: si does not usually trigger the subjunctive in this kind of sentence.

Because after a preposition like antes de, Spanish uses the infinitive when there is no new subject explicitly introduced.

So:

• antes de llegar a la playa = before getting to the beach / before arriving at the beach

This is shorter and very natural.

If you want to state the subject more clearly, then Spanish uses:

• antes de que + subjunctive
• antes de que lleguemos a la playa = before we get to the beach

Both can work, but the infinitive version is more compact.

It is understood from context, and it is most naturally we or I/we, not the ice cream.

So the idea is:

• If the ice cream melts before we get to the beach...

Spanish often leaves this kind of subject unstated with the infinitive when it is easy to infer.

If you want to remove any possible ambiguity, you can say:

• antes de que lleguemos a la playa

That makes the subject fully explicit.

Because llegar normally takes the preposition a to mark the destination.

• llegar a casa = to arrive home
• llegar al hotel = to arrive at the hotel
• llegar a la playa = to arrive at the beach

So a here does not mean to in a word-for-word way so much as it is the preposition required by llegar.

Because Spanish often uses a noun without an article when speaking about something in a general or non-specific way, especially after verbs like comprar.

So:

• comprar fruta = buy fruit / buy some fruit

This sounds natural and general.

If you said comprar la fruta, that would usually sound more specific, as if you mean a particular fruit already known in the conversation.

Because otro normally replaces un/una.

So in Spanish you say:

• otro postre = another dessert

not

• un otro postre

This is the normal rule:

• otro libro = another book
• otra idea = another idea
• otro postre = another dessert

But compare:

• el otro postre = the other dessert

That has a different meaning.

En lugar de means instead of.

Yes, you could also say en vez de in this sentence:

• prefiero comprar fruta en lugar de otro postre
• prefiero comprar fruta en vez de otro postre

Both are natural. In many situations they are interchangeable. En lugar de can sound a little more neutral or slightly more formal, while en vez de can sound a bit more everyday, but the difference is small.

Because after preferir, Spanish usually uses an infinitive when the same person is doing both actions.

• Prefiero comprar fruta = I prefer to buy fruit

The subject of prefiero and comprar is the same person: I.

If the subject changes, Spanish usually switches to a clause with que:

• Prefiero que compres fruta = I prefer that you buy fruit

Yes, it is normal and recommended here.

When a conditional clause beginning with si comes first, Spanish usually separates it from the main clause with a comma:

• Si el helado se derrite..., prefiero comprar fruta...

This works like English punctuation in similar sentences.

If the main clause came first, the comma is often omitted:

• Prefiero comprar fruta si el helado se derrite antes de llegar a la playa.

In the most natural reading, it goes with se derrite:

• If the ice cream melts before we get to the beach, I prefer to buy fruit...

So it gives the time frame for the melting.

Because the meaning makes that clear, native speakers understand it easily. But if you want to make it more explicit, you could say:

• Si el helado se derrite antes de que lleguemos a la playa, prefiero comprar fruta...

That version leaves no doubt.

Here, la playa is the normal form because playa is a countable noun and you are referring to a specific destination: the beach.

• llegar a la playa = arrive at the beach

Spanish often uses the definite article where English might also use one, and this is a very standard example. Saying just llegar a playa would not be correct in normal Spanish.

Because Spanish often omits subject pronouns when the verb already makes the subject clear.

• prefiero already tells you the subject is yo.

So:

• prefiero comprar fruta

naturally means:

• I prefer to buy fruit

You could add yo for emphasis or contrast:

• Yo prefiero comprar fruta, pero él prefiere helado.

But in a neutral sentence, leaving it out is more natural.