I remember reading somewhere (I think in the wiki), that sentence as
subject is not allowed.

However, I can't understand how someone can use a sentence as subject
in the first place.

Can someone write an example of a sentence as subject, so that I will
understand, what someone is supposed to avoid ?

An example, in English, of something that could be considered sentence-as-subject:

The Klingon killing the Ferengi is good.
It is good that the Klingon killed the Ferengi.

Here, it stands in for the postcedent the Klingon killed the Ferengi. In Klingon, you cannot say *QaQ verengan HoHpu' tlhIngan or *verengan HoHpu' tlhIngan QaQ 'e'. You must recast. Perhaps verengan HoHpu'mo' tlhIngan QaQ ghu' because the Klingon killed the Ferengi the situation is good or QaQ tlhIngan verengan HoHpu'ghach the Klingon's Ferengi-killing is good (this latter might be hard to parse).

-- 
SuStel
http://trimboli.name