"She reads the difficult newspapers."
Translation:Léann sí na nuachtáin dheacra.
nuachtán is not a feminine noun! Its plural, nuachtáin, takes the plural form of the adjective deacair (deacra) and lenites it, because nuachtáin ends in a slender consonant. This rule doesn't apply to singular adjectives!
Nouns ending in a slender consonant DO NOT happen to be femine!!!!!! (cailín, buachaill, feirmeoir, etc). That's even more true when the noun is plural!!!
Try using muscle memory: write in longhand on a sheet of paper twenty times “singular: an”. Turn the sheet over and write twenty times “plural: na”. Turn it over again, and in a second column write twenty more times “singular: an”. Turn it over once more, and in a second column write twenty more times “plural: na”. See if that helps you to get it.
You aren't missing out on any learning - all of the teaching content on Duolingo is available to all Duolingo users.
I'm not sure what you mean by "coping a thread". Assuming it's just a typo, you can copy the text from the website if you're using a larger screen with a mouse - selecting text and copying it seems to be disabled on smaller touch screens. Alternatively, you can copy the URL to link back to a discussion, but again, that's a function of the Web site - I don't know what your options are if you're using one of the various different phone/tablet apps to access Duolingo.
DeNTaLS-DoTS doesn't apply to adjectives.
While the Tops & Notes don't explicitly mention this exception, they do include the example of an mhairteoil dhearg.
It can be a little unsettling when Duolingo throws a new skill at your during an exercise that's ostensibly teaching something else. But I like to look at it like something that would happen if you were learning the language by living among native speakers. New concepts, words, grammatical rules would just come at you — to be absorbed, hopefully, as they arise...