Mi hermana prefiere enjuagar la fiambrera a mano porque dice que así queda más limpia.

Prefiere is the 3rd person singular present tense of preferir: (él/ella) prefiere = he/she prefers.

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

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

So Mi hermana prefiere... means My sister prefers...

• prefera is not a correct form here.
• prefería would mean used to prefer / would prefer, which changes the meaning.

After preferir, Spanish commonly uses an infinitive when talking about preferring to do an action.

So:

• prefiere enjuagar = prefers to rinse
• literally: prefers rinsing / prefers to rinse

This is very common in Spanish:

• Prefiero comer en casa. = I prefer to eat at home.
• Prefieren esperar. = They prefer to wait.

You do not need de here.
So prefiere enjuagar is correct, not prefiere de enjuagar.

Enjuagar usually means to rinse.

In this sentence, it suggests rinsing or washing out the container, especially with water. Depending on context, English might translate it as:

• to rinse
• to wash out
• sometimes to wash

So enjuagar la fiambrera a mano is something like:

• to wash/rinse the food container by hand

In everyday speech, the exact English translation may vary a bit depending on how much cleaning is involved.

In Spain, la fiambrera usually means a lunchbox or food container, often the kind used to take food to work or school.

Depending on context, an English speaker might also think of:

• lunchbox
• tupperware container
• food tub/container

It is a feminine noun, which is why the sentence says la fiambrera and later más limpia.

In Spanish, it is very common to use the definite article (el, la, los, las) when referring to a specific object, even in places where English might leave it out.

Here, la fiambrera refers to a particular container that both speaker and listener can identify.

So:

• enjuagar la fiambrera = to rinse the lunchbox / the container

Spanish often sounds more natural with the article in these cases.

A mano means by hand.

So:

• enjuagar la fiambrera a mano = to wash/rinse the container by hand

This usually implies not using a dishwasher or machine.

Other examples:

• Lavo los platos a mano. = I wash the dishes by hand.
• Está hecho a mano. = It’s handmade.

Porque (one word, no accent) means because.

That is exactly what the sentence needs:

• ...porque dice que... = ...because she says that...

Compare:

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

So here porque is correct because it introduces the reason.

Dice que simply means she says that...

After verbs like decir, if you are reporting what someone says or believes to be true, Spanish normally uses the indicative, not the subjunctive:

• Dice que así queda más limpia.
• She says that this way it comes out cleaner / is cleaner.

The subjunctive would appear in different kinds of structures, for example after negation, doubt, emotion, or certain impersonal expressions:

• No dice que quede más limpia. could appear in a different context
• Duda que quede más limpia. = She doubts it gets cleaner

But in this sentence, dice que is a straightforward statement, so the indicative is natural.

Here así means this way or like this.

So:

• dice que así queda más limpia = she says that this way it ends up cleaner

It refers back to the method mentioned before:

• washing/rinsing it by hand

So the idea is: My sister prefers to wash the lunchbox by hand because she says it gets cleaner that way.

Here quedar means something like:

• to end up
• to turn out
• to come out

So queda más limpia means:

• it ends up cleaner
• it comes out cleaner

This is very natural in Spanish when talking about the result of cleaning, cooking, arranging, etc.

Compare:

• Está más limpia = It is cleaner (describing its state)
• Queda más limpia = It gets/comes out cleaner (emphasising the result of the method used)

So quedar is a very good choice here because the sister is talking about the result of washing it by hand.

Limpia agrees with la fiambrera, which is feminine singular.

So:

• la fiambrera → más limpia
• if it were a masculine noun, you would use limpio

Examples:

• El vaso queda más limpio. = The glass comes out cleaner.
• La fiambrera queda más limpia. = The lunchbox comes out cleaner.

This is standard adjective agreement in Spanish.

Yes. Más limpia means cleaner.

Spanish often forms the comparative with:

• más + adjective

So:

• más limpio / más limpia = cleaner
• más grande = bigger
• más rápido = faster

In this sentence:

• queda más limpia = comes out cleaner / ends up cleaner

Yes, some parts could be moved, but the original order is very natural.

Original:

• Mi hermana prefiere enjuagar la fiambrera a mano porque dice que así queda más limpia.

This flows as:

1. subject: Mi hermana
2. main verb: prefiere
3. action: enjuagar la fiambrera
4. manner: a mano
5. reason: porque...

You could move a mano or así in some contexts, but the sentence as given sounds very natural and clear.

For example:

• Mi hermana prefiere enjuagar a mano la fiambrera... This is possible, but often sounds a little less neutral.

So the original word order is a good standard model to learn from.

Spanish often drops subject pronouns like ella, because the verb ending already shows the person:

• prefiere already tells you it is he/she/you formal

But here mi hermana is useful because it identifies exactly who we mean.

You could say:

• Mi hermana prefiere... = My sister prefers...

If the context were already clear, Spanish might later omit the subject:

• Mi hermana prefiere enjuagar la fiambrera a mano. Dice que así queda más limpia.

So the noun phrase mi hermana is not grammatically required in every sentence, but it is completely natural and helpful here.

Sometimes yes, depending on what you mean.

• lavar = to wash
• enjuagar = to rinse

If the idea is full washing, lavar may be more direct:

• Mi hermana prefiere lavar la fiambrera a mano...

If the speaker specifically means rinsing it out, then enjuagar is better.

In real life, there can be some overlap in translation, especially when English speakers casually say wash out for something that may technically just be rinse. The exact best choice depends on context.