What is the grammatical function of "a" in "estoy a salvo"?

Question

I've seen a few sentences like

Estoy a salvo

Estoy a gusto

But the fact that "a" is required in these sentences surprised me.

It doesn't seem like "estoy" + past participle always requires "a" in between, for example:

Estoy aburrido

Estoy cansado

At first I thought the difference is because "salvo" and "gusto" are transitive verbs and I'm the direct object. But "aburrir" and "cansar" are also transitive. Perhaps "aburrir" and "cansar" are used here in the passive voice, but "salvar" and "gustar" are not passive in the above examples?

Answer 1

In fact, "gusto" is a noun and "salvo" is an adjective. The "a" preposition is needed to form adverbial locutions with nouns or adjectives.

You can find a lot of information about adverbial locutions with the "preposition + noun" pattern in the Nueva gramática española (in Spanish), in chapter 30.16. You will find a whole lot of adverbial locutions such as:

• A bordo
• Ante todo
• Bajo control
• Con frecuencia
• Contra corriente
• De golpe
• En efecto
• ...

And in chapter 30.17 you have a list of adverbial locutions with the "preposition + adjective" and other patterns:

• A diario
• De pasada
• En frío
• Por escrito
• ...

Answer 2

"A" is a preposition, meaning at or to. I'll give you a few phrases in English that work in a similar way, to help you feel more comfortable.

at ease

I serve at the pleasure of the king

I hope this has been resolved to your satisfaction

Now let's look specifically at "a salvo." It can be used to talk about reaching safety. Thinking about it this way makes it natural to expect this preposition, a.