En cuanto encontré el indicio, resolví el acertijo.

Here en cuanto means as soon as. It highlights that the second action happened immediately after the first one.

• En cuanto encontré el indicio, resolví el acertijo. = I solved it as soon as I found the clue.
• Cuando encontré el indicio, resolví el acertijo. = When I found the clue, I solved it.

Both can work, but en cuanto more strongly suggests immediacy.

They are both in the preterite because the sentence talks about two completed actions in the past:

• first: encontré = I found
• second: resolví = I solved

The preterite is the normal tense for specific, finished events. It also shows the sequence clearly: one event happened, and then the other happened right after.

If you used the imperfect instead, the meaning would change and sound more like background, habit, or an ongoing situation rather than two completed events.

Yes. After en cuanto, Spanish uses:

• indicative for actions viewed as real or completed
• subjunctive for actions that have not happened yet or are anticipated

In your sentence, the action is completed in the past, so Spanish uses the indicative:

• En cuanto encontré el indicio, resolví el acertijo.

But for a future situation, you would normally use the present subjunctive:

• En cuanto encuentre el indicio, resolveré el acertijo.

That is a very common pattern in Spanish.

Spanish often leaves out subject pronouns because the verb ending already tells you who the subject is.

• encontré = I found
• resolví = I solved

So yo is not necessary. You could say Yo encontré... or Yo resolví..., but that usually adds emphasis, contrast, or clarification.

For example:

• Yo resolví el acertijo, no él. = I solved the riddle, not him.

In a neutral sentence, leaving out yo is more natural.

Spanish uses articles more often than English does. Here, el indicio and el acertijo refer to specific things in the situation or story.

So the sentence is talking about:

• a particular clue
• a particular riddle

If you said un indicio or un acertijo, it would sound more like you are introducing them as nonspecific items rather than referring to the known clue and the known riddle.

Both can be translated as clue, but they are not always identical.

• indicio often sounds a bit more formal or analytical
• pista is very common in everyday speech and games

Indicio can suggest a sign, trace, or piece of evidence that helps you infer something.
Pista often feels more like a practical hint that helps you figure something out.

In many contexts they overlap, but indicio can sound slightly more detective-like or inferential.

Acertijo usually means a riddle or a brain-teasing puzzle, often something you solve mentally.

Compare:

• acertijo = riddle / brain teaser
• rompecabezas = puzzle, and also literally jigsaw puzzle
• misterio = mystery

So resolver el acertijo sounds natural if the thing being solved is a riddle, enigma, or puzzle-like problem. If you were talking about a crime story or unexplained case, misterio would be more likely.

Resolver un acertijo is the most natural and standard combination.

Spanish has preferred verb-noun pairings, and this is one of them:

• resolver un acertijo
• resolver un problema

You might hear solucionar un problema very often, but solucionar un acertijo sounds less natural to many speakers. It is understandable, but resolver is the better choice here.

Yes. You can also say:

• Resolví el acertijo en cuanto encontré el indicio.

That is grammatically fine. The meaning stays basically the same.

The original order puts the time clause first, which gives slightly more emphasis to the moment of finding the clue. English does the same thing sometimes:

• As soon as I found the clue, I solved the riddle.

Putting the main clause first can sound a little more direct:

• I solved the riddle as soon as I found the clue.

There is a comma because the sentence begins with a subordinate time clause:

• En cuanto encontré el indicio, ...

When that kind of clause comes first, Spanish normally separates it from the main clause with a comma.

If the order is reversed, the comma is often omitted:

• Resolví el acertijo en cuanto encontré el indicio.

So the comma here is mainly about sentence structure, not a pause with special meaning.