When do adjectives follow the substantive?
I just commented on a question where part of the question was "sweet dreams". One answerer translated this to "dulces sueños" and I instantly wanted to "correct" this to "sueños dulces". Why? Mostly I'm very confident about placing an adjective before or after the substantive but I don't use or know of any rules about positioning.
Please can y'all provide me with some kind of platform that I can use when I need to make this decision.
Y'all know that now I live in Texas, I'm also trying to learn Texan.
4 Answers
(a) Descriptive adjectives are generally placed after the noun that they qualify:
"Prefiero la camisa azul" (not the red or white one)
"La tarántula es una creatura peligrosa" (other creatures may not be)
"Voy a cambiar los neumáticos gastados" (only the worn tyres will be changed)
"Voy a cambiar los gastados neumáticos" (non-differentiating - all tyres will be changed)
(b) Adjectives may surround the noun, with the less important adjective first:
"Este sitio es un típico restaurante turístico"
(c) Adjectives of quantity go first:
""Tengo un medio litro de gasolina"
(d) Adjectives before the noun may convey an emotional content or a subjective judgement:
"He visto un trágico accidente"
(e)Adjectives of number or sequence go first:
"El tercer mundo"
(f) Adjectives that describe an inherent quality go first:
"La blanca nieve" or "Los altos picos"
If you were to write "la nieve blanca" you would be seeking to differentiate white snow from snow of a different colour! Also peaks are inherently high and may not be differentiated from other peaks with the adjective "high"
Often it seems that adjectives before a noun carry some kind of emotional meaning so one tip is: adjectives 'after' - for non-disputable facts or descriptions; but adjectives 'before' to express how you feel about something or sometimes even how you're affected by it - eg viejo amigo vs amigo viejo.
Perhaps for this reason if you do hear it, I think it's more common to say (que tengas) 'dulces sueños' o 'lindos sueños' with the adjective first 
Although I do wonder if 'que sueñes con los angelitos' would be more a more 'native expression' (for want of a better term)?
Re adjective placement, his is a good article on the subject, I remember finding it quite helpful:
Adjectives of appreciation: By placing an adjective before the noun, you can sometimes indicate a certain amount of appreciation for that quality and/or a certain amount of emphasis. In English we sometimes do the same thing by using a word such as "really" or by a change in intonation.
Examples: Es un músico bueno, he's a good musician; es un buen músico, he's a really good musician; la hermosa vista, the beautiful view; Hollywood, la ciudad de incontables películas, Hollywood, the city of countless movies.
Reinforcing adjectives: Adjectives that reinforce the meaning of the noun, such as adjectives that "go with" the accompanying noun, often are placed before the noun. In many cases, one might also say that the purpose of these adjectives is less for describing the noun that's modified and more for conveying some sort of an emotion to it.
Examples: Una oscura noche, a dark night; el horrible monstruo, the horrible monster; la alta montaña, the high mountain; la blanca nieve, the white snow. Nondescriptive adjectives: Many adjectives other than those that describe go before the noun.
Examples: Pocos libros, few books; muchas palomas, many doves; mi casa, my house; esta mesa, this table; dos libros, two books. Sometimes these adjectives are known by other names, such as possessive pronouns and determiners.
Meaning-changing adjectives: Some adjectives change in meaning (or at least in English translation) depending on whether they're placed before or after the noun. Generally, the adjectives placed after nouns have an objective meaning or one that carries little or no emotional content, while one placed before the noun can indicate something about how the speaker feels toward the person or thing being described.
Example: Mi viejo amigo, my longtime friend; mi amigo viejo, my elderly friend. Generally, except for the meaning-changing adjectives, you won't go wrong by placing a descriptive adjective after the noun. However, as you examine Spanish writing or listen to the spoken language, you will develop a "feel" for those cases where you can or should place adjectives ahead of the noun.
I don't have a rule for you, but I hear "dulces sueños" a lot here in Guatemala, along with "Que duermas con los angelitos."
The politically incorrect answer is that in this case it doesn't change the meaning of "sweet dreams". So when talking I would not worry about which one came out of my mouth "dulces sueños" or "sueños dulces". I say "sueños dulces".
