Mi hermana prefiere una tarrina de vainilla porque dice que así el helado no se derrite tan rápido.

Prefiere is the third-person singular present tense of preferir: mi hermana prefiere = my sister prefers.

Preferir is a stem-changing verb in the present tense:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

The e in the stem changes to ie in most forms, so prefiere is the correct form here.

In Spain, una tarrina usually means a small tub, cup, or container, especially for food like ice cream.

So una tarrina de vainilla means something like:

• a tub of vanilla ice cream
• a cup of vanilla ice cream

In an ice-cream context in Spain, tarrina is very natural. It contrasts with something like:

• un cucurucho = a cone

So the sentence suggests she prefers ice cream in a cup/tub, not necessarily in a cone.

In Spanish, flavours are very often expressed with de:

• helado de vainilla = vanilla ice cream
• batido de chocolate = chocolate milkshake
• yogur de fresa = strawberry yogurt

So una tarrina de vainilla literally means a tub/cup of vanilla. In context, it clearly means a tub/cup of vanilla ice cream.

Using de for flavours is much more natural than trying to use the flavour word directly like an English adjective.

When Spanish names a material, flavour, or type, it often uses de + noun without an article:

• helado de vainilla
• tarta de queso
• zumo de naranja

Here vainilla is functioning as the flavour/type, not referring to a specific vanilla. So de vainilla is the normal form.

You would use de la vainilla only in more specific contexts, for example if you were talking about a particular vanilla already mentioned.

Porque means because.

This is one of the most common confusions for learners, because Spanish has several similar forms:

• porque = because
• por qué = why
• por que = less common combination in other structures
• porqué = a noun meaning the reason

In this sentence, porque introduces the reason:

• Mi hermana prefiere... porque dice que...
• My sister prefers... because she says that...

So porque is exactly the form you need here.

Dice que means she says that.

• dice = she says
• que = that

After verbs of saying, believing, thinking, etc., Spanish usually uses the indicative when the speaker presents the information as a fact or belief:

• Dice que el helado no se derrite tan rápido.

That is why you get:

• se derrite (indicative) not
• se derrita (subjunctive)

The subjunctive would be more likely in negative or doubtful contexts, for example:

• No dice que el helado se derrita más despacio.
  But in your sentence, she is simply stating what she says, so the indicative is natural.

Here así means something like:

• that way
• like that
• in that way

So:

• dice que así el helado no se derrite tan rápido means:
• she says that that way the ice cream doesn’t melt so quickly

It refers back to the previous idea: choosing una tarrina. In other words, she thinks that by having it in a tub/cup, the ice cream melts less quickly.

The verb here is derretirse, which means to melt.

• El helado se derrite = the ice cream melts
• Yo derrito la mantequilla = I melt the butter

This is a very useful contrast:

• derretir = to melt something
• derretirse = to melt, to become melted

So in the sentence, the ice cream is not melting something else; it is melting itself / undergoing the change, so Spanish uses se derrite.

This is a very common pattern in Spanish:

• La puerta se abre = the door opens
• El vaso se rompió = the glass broke
• La ropa se secó = the clothes dried

Tan is used with adjectives and adverbs in comparisons or in structures like not so...:

• tan rápido = so quickly
• tan grande = so big

In the sentence:

• no se derrite tan rápido means
• it doesn’t melt so quickly

If you said muy rápido, that would mean very quickly, which changes the meaning:

• no se derrite muy rápido = it doesn’t melt very quickly

That is similar, but not exactly the same as it doesn’t melt so quickly. The original sentence is comparing the speed with another possible situation.

Because in Spanish, adjectives are often used adverbially in ways that sound very natural.

So:

• rápido can mean quick or quickly, depending on context

Examples:

• Habla rápido = He/She speaks quickly
• Corre rápido = He/She runs quickly
• Se derrite rápido = It melts quickly

You may also hear rápidamente, but rápido is more common and more natural in everyday speech in many cases.

So tan rápido here is completely normal.

Yes, Spanish word order is flexible.

The sentence as written is natural:

• dice que así el helado no se derrite tan rápido

But you could also hear:

• dice que el helado así no se derrite tan rápido
• dice que el helado no se derrite tan rápido así
  although the last one is less natural in this context

Putting así before el helado highlights the idea of that way / in that way early in the clause. It connects neatly to the previous idea: choosing a tarrina.

So this word order is not random; it helps the sentence flow naturally.

The sentence is describing a general preference and a general belief, not a one-time completed action.

• Mi hermana prefiere... = my sister prefers...
• dice... = she says...
• el helado no se derrite... = the ice cream doesn’t melt...

This present tense often expresses:

• habits
• general truths
• repeated situations
• opinions that are true right now

So the sentence sounds like a normal statement about what she usually prefers and why.

Here el helado means ice cream in a general sense, even though it uses the definite article el.

Spanish often uses the definite article when speaking about things in a general way:

• El café me gusta mucho = I like coffee a lot
• La música ayuda a relajarse = Music helps you relax
• El helado se derrite rápido = Ice cream melts quickly

So el helado here does not have to mean one specific ice cream. It can mean ice cream as a general thing.

No, not in normal standard Spanish here.

You need:

• dice que... = she says that...

Spanish generally requires que to introduce the following clause:

• Dice que viene
• Cree que es verdad
• Pienso que no

English often drops that, but Spanish usually keeps que. So dice que is the normal and correct structure.