If it precedes it _may_ drop the final syllable. When _grande_ (or
_gran_) precedes it generally refers more to quality than to size, but
this rule is not strict _at all_, as much is left to the tone of the
voice and also to gesture.

When "Santo" means "holy" it is always written in full.
When "Santo" means "saint" it drops the "to" before the name following.

EXCEPTIONS--

Santo Tomas (or Tome), Santo Domingo, and Santo Torribio.

But--La isla de San Tomas (the Island of St. Thomas, West Indies).

Some adjectives alter their meaning according as they precede or follow
a noun, as--

Cierto hombre: A certain man. (Not "un cierto.")
Una noticia cierta: A certain (sure) news.

Un pobre escritor: A poor writer--of little worth.
Un escritor pobre: An impecunious writer.

Un simple favor: A simple favour.
Un favor simple: A simple favour.

Un simple soldado: A simple (plain) soldier.
Un hombre simple: A simple man, a simpleton.

"One" and "ones" after an adjective are always left untranslated. "Man"
and "woman" in the majority of cases are also omitted, as--

Tengo algunos buenos: I have some good ones.
Un frances, una francesa: A Frenchman, a Frenchwoman.

Numeral adjectives used for measurement are translated as follows--

Una plataforma de 30 pies de largo y seis pies de ancho (_also_ de 30
pies por seis): A platform 30 feet long by six feet wide.

Este tanque tiene 16 pies de profundidad: This tank is 16 feet deep.

"In" after a superlative relative is rendered by _de_, as--

Es el negociante mas prospero de la ciudad: He is the most successful
merchant in the city.

The proportional adjectives are--

El doble (the double)
El triple (3 fold)
El cuadruplo (4 fold)
El quintuple (5 fold)
El sextuple (6 fold)
El decuplo (10 fold)
El centuplo (100 fold)
Siete veces tanto (7 fold)
Ocho veces tanto (8 fold). Etc., etc.

The adjective is used also adverbially, oftener in Spanish than in
English, as--

Hablar claro, alto, bajo, fuerte: To speak clearly, loudly, softly,
strongly.

Comprar caro, barato: To buy dear, cheap.

Ir derecho: To go straight.

Tener fuerte: To hold fast.

Exclamo ufano, cortes, enfadado, bondadoso: He proudly, courteously,
angrily, kindly exclaimed.

Justo ha llegado una carta: A letter has just arrived.

Adjectives have a governing power through a preposition and then the
preposition to be used belongs to the "idiom" of the language. Practice
will make perfect.