Expressing close possession without "de"
Normally possession is expressed with the particle 的. However, in some cases you can omit 的. This happens when the possession is about:
- A close personal relationship (family and close friends)
- An institutional or organization membership
In these cases 的 should be omitted (that is, it's not just optional - it'll sound odd if you put it in).
Some examples:
- 我 妈妈。
- 我 女朋友。
- 我们学校。
- 他 爸爸。
If 的 was used in the above examples, it would create a sense of distance between the two, and imply that the relationship is not particularly close.