El acertijo está oculto detrás del rompecabezas.

They are usually not the same kind of puzzle.

• acertijo = a riddle or brain teaser, usually something you solve with thinking or wordplay
• rompecabezas = a jigsaw puzzle or sometimes a general puzzle made of pieces

So in this sentence, el acertijo is the thing being hidden, and el rompecabezas is the object it is behind.

Spanish uses estar for many states or conditions, especially when something is in a certain condition at the moment.

• está oculto = is hidden
• es oculto would sound more like is hidden by nature or is secret/obscure as a permanent characteristic, which is not the usual idea here

So está oculto is the normal way to say that something is currently hidden.

Because oculto agrees with acertijo, and acertijo is masculine singular.

Agreement in Spanish:

• masculine singular: oculto
• feminine singular: oculta
• masculine plural: ocultos
• feminine plural: ocultas

So:

• El acertijo está oculto.
• La respuesta está oculta.

It works like an adjective here, even though historically it comes from a participle.

In está oculto, the meaning is basically:

• is hidden
• is concealed

Spanish often uses estar + adjective/participle to describe a resulting state:

• está cerrado = is closed
• está roto = is broken
• está oculto = is hidden

So for a learner, it is most helpful to think of oculto here as a descriptive word agreeing with acertijo.

detrás normally needs de after it:

• detrás de = behind

So:

• detrás de el rompecabezas

But Spanish contracts de + el into del:

• de + el = del

That is why you get:

• detrás del rompecabezas

You cannot say detrás el rompecabezas in standard Spanish.

Yes, but there are differences.

• detrás de = the most neutral and common choice for behind
• atrás de = used in some regions, but less standard in many contexts
• tras = more formal or literary in many situations, though still common in some fixed expressions

So for most learners, detrás de is the safest and most widely useful option.

Yes, Spanish word order is flexible.

The original sentence:

• El acertijo está oculto detrás del rompecabezas.

A possible variation:

• Detrás del rompecabezas está oculto el acertijo.

Both are grammatical. The difference is mostly about focus or style.

• Starting with El acertijo makes the sentence feel straightforward and neutral.
• Starting with Detrás del rompecabezas emphasizes the location first.

Yes, it looks plural because of -s, but rompecabezas is often an invariable compound noun. That means the form usually stays the same in singular and plural.

• singular: el rompecabezas
• plural: los rompecabezas

You tell singular or plural mainly from the article:

• el rompecabezas = one puzzle
• los rompecabezas = more than one puzzle

Both can mean hidden, and in many situations they are very close.

• oculto often sounds a little more formal, abstract, or deliberate
• escondido often sounds a little more everyday, physical, or action-based

Examples:

• El tesoro está escondido. = The treasure is hidden.
• El significado está oculto. = The meaning is hidden.

In your sentence, both could work:

• El acertijo está oculto detrás del rompecabezas.
• El acertijo está escondido detrás del rompecabezas.

The first one sounds slightly more polished or literary.

Because Spanish accent marks show when a word is stressed in a way that does not follow the normal stress rules.

• está has a written accent to show stress on the last syllable: es-TÁ
• detrás has a written accent to show stress on the last syllable: de-TRÁS

But:

• acertijo follows normal stress rules: a-cer-TI-jo
• rompecabezas also follows normal stress rules: rom-pe-ca-BE-zas

So those two do not need written accents.

A rough pronunciation guide is:

el ah-sehr-TEE-ho eh-STAH o-kool-toh deh-TRAHS del rohm-peh-kah-BEH-sahs

A few useful notes:

• j in acertijo sounds like a strong h sound in Latin American Spanish
• ll does not appear here, but if it did, pronunciation would vary by region
• h is silent in Spanish, but there is no h in this sentence
• r in rompecabezas is a light tap here, not a strong English r

Yes. Literally, it means something is physically behind something else. But it could also be used in a more figurative or creative way.

For example, in a game, story, or mystery context, it could suggest:

• the riddle is concealed by the puzzle
• you must solve the puzzle to get to the riddle
• the riddle is metaphorically hidden behind the puzzle

So the sentence can work both literally and in a more imaginative context, depending on what is being described.