Translation:The teacher's summary is bad.
I know it's been 9 months, but maybe it'll be useful for other people who are reading this..
Elidenhaag is right when he/she said that 'van' is possessive, which means it is -his- summary. If you say "The summary -from- the teacher" without any context, it's possible that the summary may have been written by someone else. This sentence only tells you that you got it from your teacher, nothing else. Saying "-van- de leraar" implies that it's his summary, so it would be an incorrect or incomplete translation.
Hope this helps!
Yes, stripped from the context "van de leraar" can both mean "from him" and "of him".
De samenvatting van de leraar is slecht = The summary of the teacher is bad Wij krijgen een samenvatting van de leraar = We receive a summery from the teacher
I'm not a Dutch linguist, so it's hard for me to explain the rule, but my feeble attempt would be to say that in the first sentence 'van' is possessive and the second it's not.
Hope that helps!
I think maybe if we want to express that the summary are written about the teacher, we could say "De samenvatting over de leraar is slecht".
In English, I used to discuss this with my friends, and we found "double possessive" very useful then. Like: this is a summary of him AND this is a summary of his. (both are correct) The latter one express clearer that the summary is not necessarily about him.
I don't know whether Dutch has double possessive. So I guess in order to avoid ambiguity, we should carefully select the preposition.