suyo y su ?
How do you know when to use suyo vs su?
4 Answers
su is a possessive adjective and suya is a possessive pronoun.When you use suya it would probably be preceded by the words "de," or "de la." When using su, I believe that it would usually precede the noun that it modifies
It's a good thing Heidita isn't around or you'd be in the dunce corner for sure. 
su is the short form possessive adjective and suya is the long form possessive adjective.
el suyo, la suya are the possessive pronouns. ( los suyos y las suyas)
You were on the right track, however.
The short form possessive adjective goes in front of the noun and the long form possessive adjective goes after the noun.
mi libro
el libro mío
los pantalones son los suyos.
To get back to the original question: When do you use the short form possessive adjective and when do you use the long form? - I don't know. Both are grammatically correct, but I'm sure than common usage has created situations where one would be used over the other. Like always, wait for a native.
The word "suyo" can be an adjective or a pronoun, but it the adjective is apocopated (ie. shortened) to "su" when it precedes a noun.
The following words used as adjectives are shortened when they precede a noun:
- alguno - algún
- ninguno - ningún
- cualquiera - cualquier
- bueno - buen
- grande - gran
- malo - mal
- mío - mi
- tuyo - tu
- suyo -su
- uno - un
- primero - primer
- tercero - tercer
- veintiuno - veintiún
- ciento - cien
- santo - san
Commenting on Quentin's entry gave me impetus to look up these two words in my Gran Diccionario Oxford. Here is what I found for suyo, -ya:
suyo -ya adjectivo (de él) his, (de ella) hers (de usted, ustedes) yours (de ellos, ellas) theirs.
Marta y un amigo suyo
Ser muy suyo: Eso es muy suyo. That's typical of him/her.
Es muy suya. She's an odd sort (colloquial).
And then I found the "pronombre" following that definition.
suyo -ya pronombre
el suyo, la suya etc. (de él) his, (de ella) hers; (de usted, ustedes) yours; (de ellos, ellas) theirs;
Él me prestó el suyo. He lent me his;
Hacer (una) de las suyas (familiar) - to get up to one's usual tricks.
ir a lo suyo - to look after number one;
[Examples for] lo suyo:
Tuvo que trabajar lo suyo. He had to work very hard;
Pesa lo suyo. It weighs a ton;
Salirse con la suya. To get one's own way.
su is a possessive ADJECTIVE and suya is a possessive PRONOUN.
When you use suya it should be able to stand alone such as in:
-los pantalones son suyos.
-the pants are yours.
However, suya can be used as a possessive adjective as well, and in this case it would come after the noun that it modifies:
-Son los pantalones suyos.
-They are your pants.
When using su, I believe that it would usually precede the noun that it modifies such as:
-Son sus pantalones
-They are your pants
You might want to remember that when using su, it must agree in number with the noun it modifies; whereas, suya must agree in both number and gender with the thing/things possessed.
el suyo, la suya are the possessive pronouns. ( los suyos y las suyas)
As I´m sure you are aware, as in the example I have given above, the possessive pronoun does not require the use of the definite article (i.e. you would not say los pantalones son los suyos).
As a personal pronoun suya could also be preceded by the word "de" such as with:
-Son los pantalones de suyos.
