I am quite unsure what you are talking about, since on the web platform there is a bullet list and the different words are on different lines... >_> Are you perhaps using a mobile phone? If my post is rendered without the bullet list on a mobile platform, but just as a single line, that is a very serious issue and you really should report it to the Duolingo developers.
Actually, there is a list of ALL the declensions (i.e., with many common exceptions to the regular five patterns) here: http://en.wikiversity.org/wiki/Swedish_declensions
I found this really helpful for definite and indefinite singulars and plurals, (not irregulars):
I came to ask the same question and found the answer here. Some plurals do not change (just like occasional ones in English), brev is one of these. But you would tend to use an article in the singular, but not the plural. So:
Ett brev = a letter Brevet = the letter Brev = letters Breven = the letters
There are two ways to say 'a/an' (indefinite articles) in Swedish: 'en' and 'ett'. Most nouns in Swedish use 'en' as in "en bok" while some nouns use 'ett' as in "ett brev". If 'en/ett' is in front of the noun then the meaning conveys the indefinite article. If a form of 'en/ett' is attached at the end of a noun then the meaning conveys the definite article ('the'). You kind of just have to memorize which indefinite article goes with which noun.