"My grandmother's basket is yellow."
Translation:Kosz mojej babci jest żółty.
actually it is not a good implication of free word order, phrase "koszyk mojej babci" should be treated as one "piece" when you experiment with word order, And there is only one word order that does not sound weird.
Koszyk mojej babci jest żółty
Żółty jest koszyk mojej babci - feels a bit "forced", but might work in some context or poetry
Jest koszyk mojej babci żółty - is weird, could work as a question in poetry
Koszyk mojej babci żółty jest- feels a bit "forced", could work poetry
Jest żółty koszyk mojej babci - changed meaning of the sentence to "(there) is a yellow basket of my grandmother"
Żółty koszyk mojej babci jest - again changed meaning "my grandmother's yellow basket is" (here)
Why is the translation "Mojej babci koszyk jest żółty" incorrect? The word order shouldn't matter.
No. Just no. The word order matters. Sometimes even a lot. As immery said, it is flexible, but not meaningless.
Because in Polish, the word order changes the meaning of phrase, and even the meaning of words. Sometimes it can make the phrase mean something completely different than what you intended to. Sometimes by changing the word order you get the level of control on the shades of meaning, that is probably unattainable in English language and often elusive for the speakers of non-Slavic languages.
But before you learn how to walk a tightrope, you'd better learn to walk on the ground. Before you play with the word order, you'd better learn the neural one.
P.S. The phrase "Koszyk mojej babci żółty jest." - is a Yoda Speak ;-)
Unfortunately Polish "free word order" does not mean word order doesn't matter. It is just more flexible than English.
One of the "unbreakable" rules is indicating possession as:
noun "possessed"(in required case)+noun"possessing"(in genitive).
you also never change word order in "possessive pronoun"+noun (possessed)