grammar.adjectives.position-meaning-change
Adjectives that change meaning by position
Un viejo amigo (longtime friend) vs un amigo viejo (elderly); un gran libro (great) vs un libro grande (big); pobre, único, mismo, antiguo, cierto, nuevo all shift.
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Explication
A set of adjectives changes meaning depending on whether it comes before or after the noun. Before the noun the sense is usually figurative or evaluative; after the noun it's literal and classifying.
The key pairs (before / after): gran vs grande ('great' / 'big'), viejo ('longtime' / 'elderly'), pobre ('pitiable' / 'penniless'), único ('only' / 'unique'), antiguo ('former' / 'ancient'), nuevo ('another, newly acquired' / 'brand-new'), cierto ('a certain' / 'true'), mismo ('same' / '-self'), puro ('sheer' / 'pure'), medio ('half' / 'average').
So un viejo amigo is a friend of long standing, while un amigo viejo is an elderly friend; un gran libro is a great book, un libro grande a big one; el mismo día 'the same day' but el día mismo 'the very day'. When in doubt, the post-noun position gives the plain, literal meaning.
Exemples
He's a longtime client of the firm.
Région: global
He bought a brand-new car... well, another used car.
Région: global
He's an old friend, though he isn't an old man.