L'indovinello mostra che siamo fuori strada.

Because indovinello starts with a vowel, and the singular definite article becomes l' before a vowel.

So:

• il libro = the book
• l'indovinello = the riddle

This l' is used for both masculine and feminine singular nouns before a vowel:

• l'amico = the friend
• l'amica = the friend

It is masculine singular.

You can tell from:

• the article l' here, which could be masculine or feminine by itself
• the noun ending -o, which is very often masculine in Italian

Its plural is gli indovinelli.

Indovinello means riddle.

It comes from the verb indovinare, which can mean to guess. So an indovinello is something you have to solve or guess.

Because mostra is the conjugated verb, while mostrare is the infinitive.

Here the subject is l'indovinello, which is singular, so the verb is in the third person singular of the present tense:

• mostrare = to show
• mostra = shows

So:

• L'indovinello mostra... = The riddle shows...

Che means that and introduces a new clause.

So:

• mostra che... = shows that...

In English, that is often optional:

• The riddle shows that we are off track
• The riddle shows we are off track

In Italian, che is normally kept.

Siamo is the first person plural form of essere:

• sono = I am / they are
• sei = you are
• è = he/she/it is
• siamo = we are

So siamo means we are.

Because Italian often leaves out subject pronouns when the verb already makes the subject clear.

Since siamo already means we are, noi is unnecessary.

So both are possible:

• siamo fuori strada
• noi siamo fuori strada

The version with noi usually adds emphasis or contrast.

Literally, fuori strada means something like off the road or out of the road/path.

Idiomaticially, it often means:

• on the wrong track
• mistaken
• headed in the wrong direction

In this sentence, it is almost certainly idiomatic, not about physically leaving a road.

Because fuori strada works as a fixed expression.

Italian often uses expressions like this without an article, especially in idiomatic phrases.

So:

• essere fuori strada = to be off track / mistaken

You do not normally say fuori la strada here.

Because mostra che presents the following idea as something shown or established, so the indicative is natural.

Italian often uses the indicative after verbs that present something as a fact or clear evidence:

• Vedo che sei stanco. = I see that you are tired.
• L'indovinello mostra che siamo fuori strada.

The subjunctive is more common after expressions of doubt, emotion, uncertainty, or personal judgment.

Yes. That is a very natural English rendering.

Even though fuori strada literally suggests being off the road/path, the idiomatic meaning here is probably:

• we’re on the wrong track
• we’re mistaken
• we’re going in the wrong direction

So a natural English translation may not match word-for-word.

Yes, but it changes the meaning slightly.

• L'indovinello mostra che siamo fuori strada = The riddle shows that we are off track.
• L'indovinello ci mostra che siamo fuori strada = The riddle shows us that we are off track.

The version without ci is more neutral and general. The version with ci emphasizes the people being shown something.

Yes, depending on context.

The Italian present tense can correspond to:

• shows
• is showing

In this sentence, shows is the most natural English choice, but Italian mostra itself does not force that distinction.

Yes, it is very normal.

The structure is:

• L'indovinello = subject
• mostra = verb
• che siamo fuori strada = subordinate clause

This is a standard Italian sentence pattern:

Subject + verb + che-clause

So the sentence sounds natural and straightforward.