"The women are not wearing t-shirts, but shirts."
Translation:A nőkön nem póló van, hanem ing.
You need to alter the word order a little and make the shirts singular, unless each woman is wearing multiple shirts. So you can do this:
A nők nem pólót viselnek, hanem inget.
A useful guideline (that comes up in a LOT of sentences in this course) is that "not X, but Y" sentences like this one - not t-shirts, but shirts - will translate into Hungarian as nem X ..... hanem Y with the X immediately after nem and the Y immediately after hanem.
It seems that this course doesn't introduce transitive verbs for "wear" like visel and hord, and prefers to use these constructions which are more like "The women are in t-shirts" or "The women have t-shirts on."
(Apologies if I'm telling you something you already know - it looks like you probably don't need this kind of explanation. But it might be useful for someone else.)