"The boy does not know that there is a pink hippo."
Translation:Το αγόρι δεν ξέρει ότι υπάρχει ροζ ιπποπόταμος.
I would say that "The boy does not know that there is a pink hippo" is actually a specific not a generic sentence in English. It sounds like the boy doesn't know there is a pink hippo "in the zoo"/"in the garden"/"in the bathroom". To be generic you'd need to include the "such a thing as...."
However, as this a Greek course, not an English one, I take it the key point here is that if it is specific then "ένας" should be included, but if it is generic, then it should be left out? I have often found the inclusion or not of the definite article a bit confusing.
In the example you stated, both versions should have been accepted. In this case, however, it would sound unnatural if "ένα" is not omitted.
That is, if we say that the sentence here is a general statement ("...that there are pink hippos"). If it's not a general statement, but we're talking about some specific pink hippo, yes, it's better to include "ένα".