Voy a quedarme dormido en el sofá.

Voy a + infinitive is a very common way to talk about the near future in Spanish, similar to I’m going to ... in English.

So voy a quedarme dormido means the speaker expects this to happen soon, or feels it is likely to happen.

You could also use the simple future, such as me quedaré dormido, but in everyday speech voy a ... often sounds more natural and conversational.

Quedarse dormido is a very common expression meaning to fall asleep.

Even though quedarse by itself often means to stay or to remain, in this expression the whole phrase has its own meaning. It suggests that someone ends up asleep, often without really planning to.

So it is best to learn quedarse dormido as a set expression.

Because after voy a, you need an infinitive: quedar.

When a reflexive verb is in the infinitive, the reflexive pronoun can be attached to the end:

• voy a quedarme dormido

This is also possible:

• me voy a quedar dormido

Both are correct. Spanish allows the reflexive pronoun either:

• before the conjugated verb, or
• attached to the infinitive

But me quedar is not possible, because the pronoun cannot go before a bare infinitive in that position.

Yes. That is completely correct and very natural.

These two versions mean the same thing:

• Voy a quedarme dormido en el sofá.
• Me voy a quedar dormido en el sofá.

The difference is mainly word placement, not meaning.

Normally, no.

The expression is quedarse dormido, not just quedar dormido. The reflexive pronoun is part of the verb here.

So you need:

• voy a quedarme dormido or
• me voy a quedar dormido

Without me, it sounds wrong to most speakers in this context.

They are related, but not identical:

• dormir = to sleep
• dormirse = to fall asleep / to go to sleep
• quedarse dormido = to fall asleep, often with the idea of drifting off or nodding off

So:

• Voy a dormir en el sofá = I’m going to sleep on the sofa.
• Voy a dormirme en el sofá = I’m going to fall asleep / go to sleep on the sofa.
• Voy a quedarme dormido en el sofá = I’m going to end up falling asleep on the sofa.

Quedarse dormido often suggests it may happen unintentionally, or because the person is very sleepy.

Often, yes.

It does not always have to mean completely accidental, but it often gives that feeling: someone is so tired that they will likely drift off.

That is one reason this phrase is very common for situations like:

• on the sofa
• during a film
• while waiting somewhere

If you want a more neutral idea of simply going to sleep, dormirse can sometimes sound less accidental.

Because dormido agrees with the person who is falling asleep.

If the speaker is male, you say:

• Voy a quedarme dormido

If the speaker is female, you say:

• Voy a quedarme dormida

If a group is speaking, it changes too:

• Vamos a quedarnos dormidos
• Vamos a quedarnos dormidas

So dormido/dormida behaves like an adjective and must match gender and number.

In Spanish, en is the normal preposition for being on or in many places, including furniture.

So Spanish says:

• en el sofá
• en la cama
• en la silla

Even though English often uses on, Spanish commonly uses en here.

Sobre el sofá usually sounds more like above the sofa or on top of the sofa in a more literal, physical sense, so it is not the usual choice in this sentence.

Because grammatical gender in Spanish is not decided only by the final letter.

Sofá is a masculine noun, so it takes:

• el sofá
• un sofá

The accent mark is there because the stress falls on the last syllable: so-FÁ.

So this is just something you learn with the noun:

• el sofá = masculine noun
• stress on the final syllable because of the written accent

Yes. Voy a quedarme dormido en el sofá is a very natural, neutral sentence.

Spanish word order is flexible, so other versions are possible, such as:

• Me voy a quedar dormido en el sofá
• En el sofá me voy a quedar dormido

But the original version is straightforward and very idiomatic.