El cofre guarda la linterna ultravioleta.

Because Spanish nouns have grammatical gender.

• cofre is a masculine noun, so it takes el
• linterna is a feminine noun, so it takes la

This is grammatical gender, not biological sex. A cofre is an object, but it is still treated as masculine in Spanish grammar.

So:

• el cofre
• la linterna

You usually need to learn the noun and its article together.

Guardar is the infinitive, meaning to keep, to store, or to put away.

In the sentence, the verb is conjugated to match the subject el cofre:

• yo guardo = I keep/store
• tú guardas = you keep/store
• él/ella guarda = he/she keeps/stores

Since el cofre is a third-person singular subject, Spanish uses guarda.

So:

• El cofre guarda... = The chest keeps/stores...

Here, guarda means something like:

• keeps
• stores
• contains
• holds

With an object like cofre, English would often prefer contains or holds, even though Spanish uses guardar very naturally.

So depending on context, El cofre guarda la linterna ultravioleta could be understood as:

• The chest stores the ultraviolet flashlight
• The chest contains the ultraviolet flashlight

Because guardar already gives the idea of keeping or storing something.

Spanish does not need a separate preposition here. The sentence structure is simply:

• subject + verb + direct object
• El cofre
  • guarda
    • la linterna ultravioleta

In English, we may sometimes say The chest keeps the flashlight inside, but Spanish often leaves that idea implied because guardar already suggests that the object is kept somewhere.

Because adjectives in Spanish usually come after the noun.

So:

• la linterna ultravioleta = the ultraviolet flashlight

This is the most normal position for a descriptive adjective in Spanish. English usually puts adjectives before the noun, but Spanish usually places them after.

Compare:

• una casa grande = a big house
• un coche rojo = a red car
• una linterna ultravioleta = an ultraviolet flashlight

It already has a form that works for both masculine and feminine singular nouns.

In this sentence:

• linterna is feminine singular
• ultravioleta is an adjective that does not change between masculine and feminine singular

So you can have:

• un foco ultravioleta
• una linterna ultravioleta

The adjective stays ultravioleta in both cases.

But in the plural, it does change:

• linterna ultravioleta
• linternas ultravioletas

So it does agree in number, even if masculine/feminine singular look the same.

Yes. In Spain, linterna is the standard word for flashlight.

For a British English speaker, it often corresponds to torch.

So in Spain:

• linterna = flashlight / torch

That is the most natural everyday word.

In this sentence, using the articles is the normal choice.

Spanish uses definite articles much more often than English does. Here, el cofre and la linterna ultravioleta sound like specific things already known in the context.

• El cofre = the chest
• La linterna ultravioleta = the ultraviolet flashlight

If you removed the articles, the sentence would sound unnatural in normal Spanish:

• Cofre guarda linterna ultravioleta ❌

So yes, the articles are needed here.

Because it is the direct object of the verb guarda.

Spanish word order here is straightforward:

• El cofre = subject
• guarda = verb
• la linterna ultravioleta = direct object

So the meaning is:

• The chest stores the ultraviolet flashlight

The chest is doing the action, and the flashlight is receiving that action.

Yes, absolutely.

That would mean:

• The chest contains the ultraviolet flashlight

In some contexts, contiene may sound a bit more literal or formal than guarda. Meanwhile, guarda can suggest keeps, stores, or has inside.

So both are possible, but they are not always identical in tone:

• guarda = keeps/stores
• contiene = contains

It is pronounced roughly ool-tra-byo-LE-ta, with the stress on le.

Because it ends in a vowel, normal Spanish stress rules would usually stress the second-to-last syllable, and that is exactly what happens here:

• ul-tra-vio-le-ta

A rough pronunciation guide for the whole sentence:

• El cofre guarda la linterna ultravioleta
• el KOH-fre GWAR-da la leen-TER-na ool-tra-byo-LE-ta

The exact sounds will depend on accent, but that is a good approximation for Spain Spanish.