Eu só conto o segredo, desde que você prometa ficar em silêncio.

Here desde que means provided that, as long as, or on condition that.

So the structure is:

• Eu só conto o segredo = I only tell the secret
• desde que você prometa ficar em silêncio = provided that you promise to keep quiet

Be careful: desde que can mean different things in other contexts. It does not always mean provided that.

Because desde que here introduces a condition, and that triggers the subjunctive in Portuguese.

So:

• você promete = you promise — indicative, a plain statement
• você prometa = you promise — subjunctive, used after expressions of condition, doubt, desire, etc.

This is a very common pattern:

• desde que você faça...
• desde que ele venha...
• desde que nós possamos...

So desde que você prometa is the natural grammatical choice.

Portuguese often uses the present tense to express something that is true in general, immediate, or dependent on a condition.

So Eu só conto o segredo... can naturally mean:

• I only tell the secret...
• I’ll only tell the secret...

The exact English translation depends on context.

In this sentence, the present tense sounds natural because the speaker is stating a condition for telling the secret.

Só here means only.

In Eu só conto o segredo, it limits the action:

• I only tell the secret...
• in natural English: I’ll only tell the secret...

Position matters a little. Putting só before the verb is very common:

• Eu só conto... = I only tell...

You could also hear somente instead of só, but só is much more common in everyday speech.

Portuguese uses definite articles more often than English.

So o segredo literally means the secret, and that sounds natural in Portuguese even where English might sometimes say just secret in a different structure.

Compare:

• contar o segredo = tell the secret
• guardar segredo = keep a secret / keep quiet about something

In this sentence, o segredo is the normal choice.

Because você takes third-person singular verb forms in Portuguese.

So:

• tu prometes / tu prometas
• você promete / você prometa

That is standard Brazilian Portuguese grammar. Even though você means you, the verb agrees as if it were he/she grammatically.

Literally, ficar often means to stay, to remain, or sometimes to become, depending on context.

So ficar em silêncio is literally something like:

• to remain in silence
• to stay silent

In natural English, the best translation is usually:

• to keep quiet
• to stay silent

It is a very common expression in Portuguese.

Yes, in many situations you could say:

• Eu só conto o segredo se você prometer ficar em silêncio.

That means basically the same thing: I’ll only tell the secret if you promise to keep quiet.

The difference is mostly nuance:

• se = if
• desde que = provided that / as long as

Desde que can sound a bit more formal or more explicitly conditional.

Also notice the verb difference:

• after se, you would normally say prometer? Actually no — in this sentence, after se referring to a likely future condition, standard Portuguese is se você prometer
• after desde que, you use the subjunctive: você prometa

So the grammar changes with the connector.

Yes, and that is often what people would say if they want to make the listener explicit:

• Eu só te conto o segredo, desde que você prometa ficar em silêncio.

That means I’ll only tell you the secret, provided that you promise to keep quiet.

Without te, the sentence is still grammatical. It just does not explicitly say to you.

Contar is very natural with segredo.

Common combinations are:

• contar um segredo / contar o segredo = tell a secret / tell the secret
• revelar um segredo = reveal a secret
• dizer um segredo = possible, but less idiomatic in many contexts

So contar o segredo is the most natural everyday phrasing here.

The comma is natural and common here, especially in careful writing.

• Eu só conto o segredo, desde que você prometa ficar em silêncio.

In informal writing, some people might omit it:

• Eu só conto o segredo desde que você prometa ficar em silêncio.

Both can appear, but the version with the comma clearly separates the main clause from the conditional clause and is a safe choice for learners.

No. That is an important point.

Desde que can have different meanings depending on context.

For example:

• Desde que cheguei aqui, estou cansado. = Since I arrived here, I’ve been tired.

But in your sentence:

• desde que você prometa...

it means provided that, and the use of the subjunctive helps signal that conditional meaning.

So context and verb mood matter.

Yes:

• Eu somente conto o segredo, desde que você prometa ficar em silêncio.

This is grammatical, but it sounds more formal or more written.

In everyday Brazilian Portuguese, só is much more common and natural.

Because the idea here is not describing someone as a generally quiet person. It is about their behavior in this situation: remaining silent.

Compare:

• ser silencioso = to be a quiet/silent person by nature
• estar silencioso = to be silent at the moment
• ficar em silêncio = to remain silent / keep quiet

In this sentence, the speaker wants the other person to keep quiet from that point on, so ficar em silêncio fits very well.

It sounds fairly neutral and natural.

Parts of it feel everyday:

• Eu só conto...
• ficar em silêncio

The connector desde que can sound a little more formal or careful than se, but the whole sentence is still perfectly natural in normal Brazilian Portuguese.

A slightly more casual version might be:

• Eu só conto o segredo se você prometer ficar quieto.

But your original sentence is absolutely fine.