Reminder: The principal function of adjectives is to modify or otherwise describe nouns. Therefore, the number (singular or plural) and the gender (masculine or feminine) of an adjective depends on the noun involved. As you know, the gender of a noun often has nothing to do with the concept of male vs. female but is rather the result of centuries of grammatical and spelling conventions.

Some other adjectives of nationality which end in a consonant include:

alemán (German)             holandés (Dutch)
escocés (Scottish)          inglés (English)
francés (French)            portugués (Portuguese)

Note that adjectives ending in -ense do not have special feminine forms: la(s) estadounidense(s), la(s) canadiense(s).

Adjectives which end in -dor in the masculine singular likewise have feminine forms with -a:

hablador, habladora, habladores, habladoras - talkative

Adjectives with shortened or contracted forms. Several adjectives are somewhat analogous to the indefinite article un and have a special form used when the adjective precedes a masculine singular noun:

alguno:    algún  alguna   algunas    (some, any) 
ninguno:   ningún ninguna (ningunos ningunas)  (no, none) 
primero:   primer primera  primeros primeras   (first) 
tercero:   tercer tercera  tercero  terceras   (third) 
bueno:     buen   buena    buenos   buenas     (good) 

• ¿Hay algún estudiante aquí que sea buen jugador?
  Is there any student here who is a good player?

However, if the adjective occurs anywhere other than immediately before a masculine singular noun, the long form is used:

• No vi hombre alguno allí.
  I didn't see a single man there.
• Es un libro muy bueno.
  It's a very good book.
• ¡Buena idea!
  Good idea!

Note that ninguno is normally used in the singular unless the noun modified is used exclusively in the plural.

• No hay ningún comunista aquí.
  There aren't any Communists here.

Special cases:

The word grande is shortened to gran before a singular noun, even a feminine noun:

• Es una gran idea.
  It's a great idea.

Santo when used to mean “Saint” before male's name, is shortened to San unless the name begins with a To- or Do-:

San Pedro      Saint Peter  
San Francisco  Saint Francis  
Santo Tomás    Saint Thomas  
Santo Domingo  Saint Dominic  

Demonstrative adjectives are used instead of articles to “point out” nouns in terms of their physical relationship to the speaker: este/esta/estos/estas (this or these, for things near the speaker), ese/esa/esos/esas (that or those, for items somewhat further from the speaker, or close to the person spoken to), and aquel/aquella/aquellos/aquellas (that or those for items in the distance). NOTE THAT THE MASCULINE SINGULAR FORMS END IN AN -e OR A CONSONANT, NOT AN -o.

• En aquel instante este chico hablaba con esas mujeres.
  At that instant this boy was talking with those women.

Note: The demonstratives —particularly ese— are sometimes placed after the noun for a pejorative effect.

• No me gusta el libro ese.
  I don't like that (worthless) book.

Note: Pronouns can be created from these forms. These pronouns are identified by a written accent mark over the stressed syllable for masculine and feminine forms to distinguish them from the adjective forms (for example, éste, ésta, éstos, éstas. The neuter forms (esto, eso, aquello) are not written with accent marks since there is no corresponding adjective to be confused with .

• ¿Qué es eso que tienes en la mano?
  What is that you have in your hand?
• Aquello que nos dijeron es rídiculo.
  That (stuff) they told us is ridiculous.

Possessive adjectives can be divided into two groups: the non-stressed and the stressed forms.

The non-stressed possessives are only used in front of nouns. The forms are:

 
mi(s)      nuestro(s)
tu(s)      vuestro(s)
su(s)      su(s)

They must agree in number (and in gender for nuestro and vuestro) with the noun, not the person or pronoun to which the refer:

• mis libros
• nuestra madre

Remember that su(s) is ambiguous; su casa = your [usted] house, his house, her house, its house, your [ustedes] house, their [male and/or female] house.

• A mi gato no le gustan tus perros.
  My cat doesn't like your dogs.
• Sus caballos arruinaron nuestras flores.
  Your [formal] / his / her / their horses ruined our flowers.

The stressed possessives are:

mío       nuestro
tuyo      vuestro
suyo      suyo

They used after nouns or by themselves or can be used with articles as pronouns:

• Ésa es la casa mía.
  That is my house. [Or: That's the house that belongs to me.]
• Esa casa es mía.
  That house is mine.
• Mi casa es más grande que la tuya.
  My house is larger than yours.

Note that the forms su and suyo are ambiguous and for clarity or emphasis sake may be replaced by the expressions de usted, de él, de ella, de ustedes, de ellas, and de ellos. For example:

• La casa de él es roja, pero la de ella es blanca.
  His house is red, but hers is white.
• (Su casa es roja pero la suya es blanca.)
  (Virtually meaningless.)

Depending on the type of adjective and the context, it may occur either before or after the noun it modifies. Most frequently, adjectives follow the noun unless they are limiting adjectives or used metaphorically or modify a noun which is one-unique.

Limiting adjectives (adjetivos determinativos) are normally are placed in front of the noun. These include adjectives which indicate quantity [e.g., mucho(s), poco(s), cuanto(s), todo(s), dos, etc.], articles (el, la, un, una, etc.), unstressed possessives (mi, tu, su, etc.), demonstratives (este, ese, aquel, etc.), and moral qualifiers (buen, mal, etc. if not preceded by adverbial modifiers such as muy) and particularly the comparative/ superlative forms such as mejor, peor):

• Pocos mexicanos han leído mis libros.
  Few Mexicans have read my books.
• Estos regalos son para los mejores niños.
  These gifts are for the best children.
• Todas las chicas son alumnas muy buenas.
  All the girls are very good students.

Descriptive adjectives (adjetivos calificativos). Adjectives which describe nouns —giving characteristics, for example color and size— normally are placed after the noun.

• ¿Puedes prestarme tu falda roja?
  Can you loan me your red skirt?
• El chico alto es mi sobrino.
  The tall boy is my nephew.
• Los estudiantes brillantes salieron bien en los exámenes.
  The brilliant students did well on the tests. (The others didn't do so well.)

It's important to realize that the final (or last or post-) position in Spanish is generally the position reserved for the element which is the most stressed or most important or most distinctive; when an adjective occurs in the post-position it has the effect of distinguishing the noun involved from other such nouns. In the the last sentence given above the phrase estudiantes brillantes occurs, with brillantes in the post-position. This implies a contrast with other estudiantes which are not brillantes (and the sentence then implies that the not-so-brilliant students did not do so well on the test). In contrast, if we wish to indicate that ALL the students were brilliant (and hence all did well on the test), or if we want to characterize the entire group as being brilliant, we would place the adjective before the noun:

• Los brillantes estudiantes salieron bien en los exámenes.
  The brilliant students did well on the tests. [That is, (all) the students —all of whom were brilliant— did well.]

Note that when a given noun is unique or one-of-a-kind, the post-position in normally required for adjectives:

• Mi esposa hermosa
  My beautiful wife: I have more than one wife; I'm referring to the one who is beautiful.
• Mi hermosa esposa
  My beautiful wife: I only have one wife, and she is beautiful.

Several descriptive adjectives change their meaning depending on whether they are used before or after the noun:

before                           after 

antiguo  former, ex-             ancient, old  
gran, grande  great              big, large  
medio  half (a)                  average  
nuevo  new, different            (brand) new  
pobre  poor, unfortunate         poor, penniless  
puro  pure (just, merely, all)   pure (clean, uncontaminated) 
viejo  old, long-standing        old (in age), elderly 
único  only                      unique  

Examples:

mi antiguo maestro - my former teacher
mi maestro antiguo - my ancient teacher

un gran presidente - a great president
un president grande - a big president (large man)

media botella - half a bottle
una botella media - an average bottle

mi nueva casa - my new house [we just moved into an old house, but it's new or different for us]
mi casa nueva - my brand-new house

una pobre mujer - an poor (unfortunate) woman
una mujer pobre - a poor (penniless) woman

pura leche - mere (or just) milk [for example, not alcohol]
leche pura - pure ( or uncontaminated) milk

un viejo amigo - an old (long-standing) friend
un amigo viejo - an old (elderly) friend

el único ejemplo - the only example
un ejemplo único/span> - a unique example