Amazing that, as a good Portuguese speaker, I always get to mix up muy /muj/ and mucho /mutʃo/ when writing in Spanish, even if the distinction is obvious (to say the least). In Portuguese, both translate as muito /mu͠jtu/.
- Está muy frío. = Está muito frio.
- Hace mucho frío. = Faz muito frio.
Although found in Portuguese, mui /mu͠j/ (or /muj/) is rare and subject to regional and/or stylistic variations (though it apparently only occurs indeed before adjectives, adverbs and verbs). It was apparently more common in older stages of the language, though, and I’ll check my books on Old Portuguese for that later.
Fortress of Solitude IV 