Use these pronouns with definite articles. You may also need to use the preposition "de" to clarify the message. See the following example:
- Te presento a Ana y Nacho. Hoy es su cumpleaños.
(Let me introduce you to Ana and Nacho. Today is HER birthday.)
- ¿De quién es el cumpleaños? ¿El de ella o el de él?
- El de Ana.
In English, possessive pronouns indicate the gender. In Spanish the possession can be more ambiguous if it is not clarified.
********
The neuter possessive "lo" is used to refer to a concept or idea:
- Preocúpate de lo tuyo; yo, de lo mío.
(Worry about what is yours and I will worry about what is mine.)
- Yo solamente quiero lo mío.
(I only want what is mine.)