Why is bonito before the noun?
There is no proper explanation. All you need is practice to get this well.
why is it "hay un bonito parque" and not "hay un parque bonito"?
agradable, here, means cosy, comfortable. In spanish you do not say: "hay un parque muy agradable" it sounds weird..you´d better say: "El parque es muy agradable, porque hay muchos bancos y tranquilidad"
Can´t "agradable" count as nice?
I've only heard "agradable" used for actions or people, but my experience is quite limited.
One would think so, but Duo's ways are mysterious. ;) Agradable = nice, bonito = pretty, I think
Why not 'bueno parque' instead of ' buen parque`? Not really sure about the difference between buen x bueno...
When used directly before a noun, the masculine form of the adjective "bueno" simply shortens to "buen". There's no difference in meaning
How can I remember that parque is masculine? Any suggestions?
Maybe since it ends with an e, and in Spanish that can denote masculine (e.g. este parque)
Why can't I use Amable in this case?
Amable is the friendly kind of nice, not the kind you would use for a sentence like this.
How about: Hay un parque suave....?