Mi madre quiere comprar una mochila resistente al agua para el viaje.

In Spanish, a possessive adjective like mi usually replaces the article, so you say mi madre = my mother, not the my mother.

• mi madre = my mother
• tu madre = your mother
• su madre = his/her/your mother

Using la before mi madre would sound wrong in standard Spanish.

This is a very common Spanish structure:

• querer + infinitive = to want to do something

So:

• quiere = she wants
• comprar = to buy

Together, quiere comprar means she wants to buy.

Spanish does not need a separate word for to between wants and buy here. The infinitive comprar already carries that idea.

Because the subject is mi madre, which is third person singular: she.

The verb querer changes depending on the subject:

• quiero = I want
• quieres = you want
• quiere = he/she wants
• queremos = we want
• quieren = they/you all want

So Mi madre quiere... = My mother wants...

Una is the indefinite article meaning a or an.

• una mochila = a backpack

Spanish often uses articles where English does too, and here it is completely natural to say una mochila because it means one unspecified backpack.

Also, mochila is a feminine singular noun, so the article must be una, not un.

The word mochila ends in -a, which is often a sign that a noun is feminine.

So you get:

• la mochila = the backpack
• una mochila = a backpack

Like many feminine nouns, it takes feminine articles and adjectives when those adjectives change form.

In Spanish, descriptive adjectives often come after the noun.

So:

• una mochila resistente = a sturdy/resistant backpack

This is more typical in Spanish than the English order resistant backpack.

In this sentence, the full description is:

• una mochila resistente al agua = a water-resistant backpack

Not all Spanish adjectives change form for masculine and feminine.

Resistente is an adjective ending in -e, and adjectives of this type usually have the same form for both masculine and feminine singular nouns.

So you can say:

• un bolso resistente
• una mochila resistente

The adjective only changes for number when needed:

• una mochila resistente
• dos mochilas resistentes

Al is a contraction of:

• a + el = al

So:

• resistente al agua literally means something like resistant to water

This is the normal Spanish expression for water-resistant.

A key thing to notice: agua is a feminine noun, but in the singular it often uses el after certain words for pronunciation reasons:

• el agua
• el aula

So a + el agua becomes al agua.

This is a very common question.

Agua is grammatically feminine, but because it begins with a stressed a sound, Spanish usually uses el instead of la in the singular to avoid an awkward sound.

So you say:

• el agua fría
• el aula pequeña

But the adjective still shows that the noun is feminine:

• agua fría, not agua frío

In the plural, it goes back to the normal feminine article:

• las aguas

Not always.

• resistente al agua usually means water-resistant: it can handle some water, but not necessarily complete or prolonged exposure.
• impermeable often means waterproof: water should not get through.

So for a backpack:

• mochila resistente al agua = good against rain or splashes
• mochila impermeable = more strongly waterproof

In many everyday situations, though, people may use these terms a bit loosely.

It tells us the purpose of buying the backpack.

So:

• para el viaje = for the trip

The idea is:

My mother wants to buy a water-resistant backpack for the trip.

It most naturally describes why she wants the backpack, not the verb quiere by itself.

Using el viaje suggests that the trip is a specific trip already known from the context.

• para el viaje = for the trip
• para un viaje = for a trip

So the sentence sounds like the speaker and listener already know which trip they mean.

Yes, but the original order is the most neutral and natural.

Standard version:

• Mi madre quiere comprar una mochila resistente al agua para el viaje.

You could move some parts for emphasis, but it may sound less neutral:

• Para el viaje, mi madre quiere comprar una mochila resistente al agua.

That is still correct, but now para el viaje is being highlighted.

A more informal version might use mamá instead of madre:

• Mi mamá quiere comprar una mochila resistente al agua para el viaje.

Both are correct, but:

• madre = more neutral / slightly more formal
• mamá = more everyday / more personal in many contexts