El acertijo incluye un indicio.

El is the masculine singular definite article, meaning the.

It is used because acertijo is a masculine singular noun:

• el acertijo = the riddle

In Spanish, nouns normally need an article more often than in English.

Acertijo is masculine, so it uses el and other masculine words:

• el acertijo
• un acertijo

A useful clue is the ending -o, which is often masculine, though not always. Here it is masculine, so el is correct.

Not exactly.

• acertijo usually means riddle: a question or statement with a hidden answer
• rompecabezas usually means puzzle, especially a jigsaw puzzle or a brain teaser more generally

So acertijo is the more specific word here.

Because incluye is the third-person singular form of the verb incluir in the present tense.

The subject is El acertijo, which is it, so Spanish uses the él/ella/usted form:

• yo incluyo = I include
• tú incluyes = you include
• él/ella incluye = he/she/it includes

So:

• El acertijo incluye... = The riddle includes...

It is mostly regular, but it has a spelling pattern that learners should notice.

Present tense:

• yo incluyo
• tú incluyes
• él/ella incluye
• nosotros incluimos
• ellos incluyen

The y appears in several forms: incluyo, incluye, incluyen.
This is normal for verbs ending in -uir that are not -guir verbs, such as:

• construir
• contribuir
• incluir

Because indicio is a masculine singular noun.

So:

• un indicio = a clue / a hint / an indication

If it were feminine, it would use una, but here the correct form is un.

They are similar, but not always identical.

• indicio often means indication, sign, or clue
• pista often means clue, especially in mysteries, games, or investigations

In many contexts, both can be translated as clue, but indicio can sound a little more like a sign or piece of evidence, while pista is often the more everyday word for a clue.

So this sentence could feel slightly more formal or literary than:

• El acertijo incluye una pista.

Because Spanish often leaves out subject pronouns when the verb form already shows who or what the subject is.

In English:

• It includes a clue.

In Spanish:

• Incluye un indicio.

The verb incluye already tells you the subject is he/she/it in singular form.
Since the full subject El acertijo is already there, adding a pronoun would be unnecessary.

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

• El acertijo incluye un indicio.

You could also say:

• Un indicio está incluido en el acertijo.

That means roughly A clue is included in the riddle, but it is more passive and less direct.

Spanish allows some flexibility in word order, but subject + verb + object is the clearest basic pattern here.

A simple approximation:

• acertijo ≈ ah-ser-TEE-ho
• indicio ≈ een-DEE-syo in much of Latin America

A few notes:

• In most of Latin America, c before e or i sounds like s
• j sounds like a strong h
• The stress is:
  • acertijo
  • indicio

Because the personal a is usually used with direct objects that are people or sometimes personified beings.

Examples:

• Veo a María.
• Escucho a mi profesor.

But indicio is a thing, not a person, so there is no a:

• incluye un indicio

Yes. In natural English, includes and contains can both work here.

So these are all reasonable translations:

• The riddle includes a clue.
• The riddle contains a clue.
• The riddle has a clue in it.

The exact best translation depends on context, but the Spanish grammar stays the same.

You would change the nouns, articles, and verb as needed.

Examples:

• Los acertijos incluyen un indicio.
  = The riddles include a clue.

• El acertijo incluye indicios.
  = The riddle includes clues.

• Los acertijos incluyen indicios.
  = The riddles include clues.

Notice:

• el → los
• acertijo → acertijos
• incluye → incluyen
• indicio → indicios