Natt = Singular; indefinite form Natten = singular; definite form
Nätter = Plural; indefinite form Nätterna = Plural; definite form
Things that end with [-na] & [-en] or [-n] usually implies definite form.
Difference between indefinite and definite are with and without brackets as following:
Bil(en) : (The) Car [Singular] Båt(en) : (The) Boat [Singular]
Låga(n) : (The) Flame [Singular] Anda(n) : (The) Breath/spirit [singular]
Låder(na) : (The) Boxes [Plural] vantar(na) : (The) Gloves [Plural]
Nätter = nights, natten = the night & nätterna = the nights. If you are using Duolingo on a mobile device, then you may not have access to the grammar lessons. To get access, you must sign into Duo via the web - either on your mobile device or a computer. Then each set of exercises will have a little lightbulb symbol. Click on this to get to the grammar lessons.
Because adjectives must agree with number (like e.g. Spanish but unlike English). Usually the base form of the adjective takes an '-a', and so 'svarta' is the plural form of 'svart'. There are also irregular forms, one very common that you perhaps recognize is that 'små' is the plural form of 'liten/litet' (little).