How about this:

> 彼女たちは 日本語を 勉強したことが あります。  
> *Kanojo-tachi wa nihon-go o benkyō shita koto ga arimasu.*  
> Them girls studied Japanese at least once.

- "koto" turns every thing preceding it into a substantive (noun) clause.
- "ga arimasu" indicates that the subject exists in at least one instance.