Adjectives and adverbs interchangeable in Spanish?

Question

I have had this problem for a while, and no Spanish speaker can readily explain it:

In English, an adjectival form can only describe a noun; for a verb, you must use the adjectival form. The only exception of which I know is "I am well." Because so few English speakers have good grammar these days, "I am good" has become a colloquialism that is acceptable. But one can never say "I cook good" or "He lives happy".

But in Spanish, I see this all the time though Spanish speakers also acknowledge the rule that adverbs, not adjectives, describe verbs. In this lesson, I just saw it again:

Espero que vivas feliz en tu apartamento nuevo.

I hope you live happily in your new flat.
Any clarification of this usage would be gratefully accepted.
Allison

Answer

Hola Allison,
Yes, in Spanish we sometimes use adjectives as adverbs; the most common cases are "rápido" and "lento", for example:

El coche iba muy rápido. 
Come más lento.

The adjectival forms rápido and lento are adverbs here modifying the verb. We tend to use this form more than the adverbs rápidamente or lentamente.
But there are more cases where the adjectival forms act as adverbs. In these cases it is telling something about the verb and the noun, both. For example:
La chica vive feliz en su apartamento. 
This adverb/adjective "feliz" is telling us the girl is happy and lives happily.
Pablo salió satisfecho del examen. 
This adverb/adjective "satisfecho" is telling us Pablo was satisfied and he came out in a satisfied way.
Me miró extrañado.
Extrañado is telling us here he was surprised and looked at me in a surprised way (looking surprised)
I think ocassionally in English you can also have this if I am not wrong:
The plane flew low. 
Low being an adjective, but acting as an adverb. 
Saludos,
Inma