You now know that we generally use er as a placeholder subject in passive sentences, het in active sentences.
Unfortunately, things are slightly complicated by the fact that some passive sentences require het instead.
Er or het?
We use er if the sentence has a subject clause that refers to a speculation or assumption. The fact is merely said, told, or announced but not proved. When the subject clause is clearly a factual statement, we use het.
|Het is bevestigd dat ze broers zijn.
||It has been confirmed that they are brothers.
|Er wordt gezegd dat ze broers zijn.
||It is said that they are brothers.
|Het is aangetoond dat fietsen gezond is.
||It has been proved that cycling is healthy.
|Er wordt beweerd dat fietsen gezond is.
||It is claimed that cycing is healthy.
The rule is often violated because it is not always easy to see whether we are dealing with a factual or a hypothetical statement. A quick search on Google gave me 33,700 search hits for "het is bewezen dat" (which is correct) but also 14,300 for "er is bewezen dat".
On the next page, we will have a look at het as a subject.