～するもの　vs　～すること grammar difference N3


I'm studying for taking the JLPT N3 and I've run across this exercise:

> この用紙は必ず黒か青のペンで記入する（_____）

Between the possible answers I had these two: もの and こと. I chose もの but the
correct answer is こと.

Could anyone help me try to understand what's the difference, in this case,
between this two?

* * *

So I did a little bit more research and I found this:

Verb + ことだ indicates essential ACTIONS.  
Verb + ものだ indicates ideal and essential STATES.

So with this I pretty much understand the exercise but still, is there
anything else I should pay attention to?

