quod scrībit

[pmda]

Hoc sciŏ: quod scrībit nūlla puella tibi!

[This I know: no girl has written to you]

This is Martial:... What I want to know is what is the purpose of 'quod' here?

It does not seem to add to the meaning or have any purpose.

[Sceptra Tenens]

It's basically an alternative to the AcI - I don't think it was widely used in formal contexts.

[adrianus]

Nescio tam multis quid scribas, Fauste, puellis.
Hoc scio, quod scribit nulla puella tibi.

"I don't know, Faustus, what you write to so many girls.
This I do know for a fact, that no girl writes to you."

quod [indicativo cum modo] = "[for a fact] that"

[pmda]

Gratias tibi ago.