Isn't gamle plural while fiskeren is singular? therefore the adjective and noun do not agree with each other. Thus, gammel should be used with fiskeren.
En gammel fisker
Den gamle fiskeren
De gamle fiskerne
It is correct. :)
"Gamle" is used with both singular and plural words. You can say "den gamle fiskeren" (=the old fisherman) and "de gamle fiskerne" (=the old fishermen). Since it is one fisherman in particular (definite), "gamle" is correct. If it was just a random fisherman, you would use "gammel": "En gammel fisker" (=an old fisherman).
Why is the translation: "The old fisherman has great grandchildren" not correct?
"Great-grandchildren" translates to "oldebarn". As a translation of "barnebarn", you want just "grandchildren".
Why is the translation 'that old fisherman has grandchildren' not correct and instead, you're supposed to write 'the old fisherman'? Isn't the translation of 'den + subject+en' that rather than the?