Okay thank you! (For both comments! So, is this right? Ledig- relating to if something is available to have or not, fri- the state of not being controlled (if you know what I mean), and gratis- relating to money.)
Second question: Would you explain to me how adjective conjugation works? Thanks.
It's hard to tell without context, but I think that here "plats" might refer to a place as in a spot, for example the chair next to you or the place next to you on a blanket. "Ställe" more refers to "place" as in a wider area, such as a building or a park. Note that "plats" can refer both to a smaller spot and a wider area, depending on context.
Those two in particular are not interchangeable. It depends on the gender of the word: "den här" or "denna" for en-words and "det här" or "detta" for ett-words. As for the difference between "den här" and "denna", "den här" uses the definite form of the noun, so that would be "den här platsen" like in Duo's sentence. "Denna" does not use the definite form, so that would be "denna plats". It also sounds quite a bit more formal than "den här", so if you used it in normal conversation you might get some odd looks.