"The boy says good morning."
Translation:Chłopiec mówi dzień dobry.
Well, it's a close-mid central unrounded vowel ;) It's a totally obvious sound to a Polish speaker, but apparently a hard one to pronounce or even perceive to most learners.
You can check here: https://en.wikipedia.org/wiki/Close-mid_central_unrounded_vowel and see if maybe something, especially in the "Occurence" section will help you. Otherwise, I guess you just need to listen a lot.
It's the same verb, just different aspects.
So it appears that in Serbo-Croatian the present stem conjugation of perfective verbs indicates regularly completed actions (present tense).
However, the present stem conjugation of perfective verbs in Polish explicitly refers to completed actions in the future, that's why powiedzieć can't be used here.
Another example using Serbo-Croatian cognates:
Piszę list - I write / I am writing a letter.
Napiszę list - I will write / I will have written a letter.