"Things are not so easy."
Translation:Le cose non sono così semplici.
I think it's because the singular forms 'facile' and 'semplice' end in 'e' - so to become plural, the 'e' becomes 'i'. The same applies to nouns which are feminine but end in 'e' - the 'e' becomes 'i' in the plural. eg. 'la lezione' - the lesson - becomes 'le lezioni' - the lessons.
This is because the equivalent adjective of numerous is "numeroso" and as we know, for such that end in o, they change their ending depending on gender and number.
Therefore, since "domanda" is feminine then it would take up "numerosa" if in singular form......But since in your example it's in plural form then we use "numerose" which is the plural feminine form.
When a noun begins a sentence, you always use the definitie article with that noun, even if it's not used in English. It's less clear whether an article is required when nouns appear inside or at the ends of sentences. Usually they are, but not always, and sometimes the grammar says not to use an article at all, e.g., sono insegnante = "I am a teacher".
It seems weird to me that languages like Italian and Spanish can so easily dispense with subject pronouns (io sono insegnante is viewed as somewhat redundant, though not incorrect) but require the opening definite article for nouns. But that's just the way things are.
All languages could easily be simplified, but then nobody would understand each other.
And of course the hover hints say facile instead of facili. I wasn't sure whether facile might be invariable, but found out that, despite Duo's hint that it is invariable, Duo marked me wrong for taking the lead from the hint provided. Now all I have to do is make sure I remember this correctly.
Duo's use of incorrect answers as a teaching mechanism seems perverse.
I agree that "simple" and "easy" are not the same - running a marathon is very simple but it is far from easy!
BUT - your answer is wrong because "facile" should be plural to agree with the noun, "cose", and because both the masc and fem singular , "facile", ends in -e the plural for both masc and fem is -i ("le cose non sono cosi facili")
"Le cose non sono cosi facile": EASY IN ENGLISH MEANS = FACILE In Italian; FACIL in French; and ALSO IN SPANISH= fácil., Aand in Portuguese: fácil IIn ALL FOUR ROMANCE LANGUAGES the first and most used translation for "EASY" IS : Italian:facile; French: facil; Spanish:fácil; Portuguese: fácil; Therefore: SEMPLICE IS NOT THE BEST TRASLATION for 'easy'. simple translation in 4 languages is as follows: It. semplice; French: simple; Spanish sencilla; Portuguese: simples.