Plural to singular

Change this to the singular form: la mesa

Change this to the plural form: las casas

Change this to the plural form: unos bolígrafos

Change this to the plural form: unos lápices

Change this to the plural form: el amigo

Change this to the singular form: unas mesas

Change this to the plural form: unos amigos

Change this to the singular form: los perros

Change this to the plural form: unos libros

Change this to the singular form: unas sillas