Breakdown of Über Sex wird in der Familie kaum gesprochen.
in
in
sprechen
to speak
die Familie
the family
über
about (a topic)
Questions & Answers about Über Sex wird in der Familie kaum gesprochen.
Why is there no grammatical subject in this sentence?
This is an impersonal passive construction. Because the active verb takes a prepositional object (über Sex) rather than a direct accusative object, there is no direct object to promote to the subject. Since the phrase Über Sex occupies the first position, the dummy subject es is dropped.
Why can't we say Sex wird über in der Familie kaum gesprochen, similar to the English "Sex is talked about"?
English allows you to separate a preposition from its object and make the object the subject of a passive sentence. German strictly forbids this. The preposition über and its object Sex must stay together as a single block.
How would you express this exact meaning in the active voice?
You would use the generic pronoun man (one/people). The active version is Man spricht in der Familie kaum über Sex. The passive version is chosen here to put all the focus on the topic rather than the people doing the talking.
Does the placement of kaum matter here?
Yes. By placing kaum (hardly) right before the participle gesprochen, it directly modifies the action of talking. If you moved it before the location, like Über Sex wird kaum in der Familie gesprochen, it would emphasize that the family is hardly the place where it happens, implying it might be discussed elsewhere.