20.which is the short form of el gato suyo?
tu gato
mi gato
su gato
vuestro gato

These sentences are a good example of the usage of the possessive adjectives in spanish.

The possessive adjectives indicate belonging (that someone or something belongs to the noun in the sentence).  

In Spanish, there are two kinds of possessive adjectives:

-Atonic possessive adjective, they are written before the noun they modify , as follows:

1st person singular Yo (I): mi/mis*

2nd person singular Tú (you): tu/tus*

2nd person singular (formal) Usted (you): su/sus*  

3rd person singular Él/Ella/eso/esa (he/she/it): su

1st person plural Nosotros (we):nuestro/nuestra or nuestros/nuestras*

2nd person plural Ustedes (you) (In latinAmerica): su/sus*

2nd person plural Vosotros (you) (In Spain): vuestro/vuestra or vuestros/vuestras*

3rd person plural Ellos/Ellas (They): su/sus*

 (*when the noun is plural)

For example: su gato (your cat), may correspond to the second person in singular (formal) usted (you) and the noun gato(cat) is in singular masculine.

-Tonic possessive adjectives, they are written after the noun they modify, as follows:

1st person singular Yo (I): mío/mía or míos/mías*

2nd person singular Tú (you): tuyo/tuya or tuyos/tuyas*

2nd person singular (formal) Usted (you): suyo/suya or suyos/suyas*

3rd person singular Él/Ella/eso/esa (he/she/it): suyo/suya

1st person plural Nosotros (we):nuestro/nuestra or nuestros/nuestras*

2nd person plural Ustedes (you) (In latinAmerica): suyos/suyas  

2nd person plural Vosotros (you) (In Spain): vuestro/vuestra or vuestros/vuestras*

3rd person plural Ellos/Ellas (They): suyos/suyas

(*when the noun is plural)

For example: el gato suyo (your cat), may correspond to the second person in singular (formal) usted (you) and the noun gato(cat) is in singular masculine.