I thought that, when a pronoun is present and the noun is plural, the adjective should end in "-en," so in this case, "interessanten." Is this just an exception, or is there something about the sentence that dictates that it be "interessante"?
Here's a great way to remember adjective ending rules, from jess1camar1e in a discussion here:http://www.duolingo.com/comment/556140 --
-Big 3 get an -e (der, die, das) der alte Mann, das kleine Kind, die schöne Frau
-Changin' gets -en (plural and case changes) den alten Mann (accusative), der schönen Frau (dative), die kleinen Kinder (plural)
-No 'the'? Adjective takes over (no 'der' word or just an 'ein') Kaltes Wetter gefällt mir nicht (das Wetter). Ein guter Mann ist schwer zu finden (der Mann).
Note changin' means the definite article differs -- has changed -- from its nominative singular form (der, die, das). So feminine accusative, for example, doesn't get an -en: "Ich sehe die schöne Katze" -- I see the beautiful cat.
Because there's no preceding article (such as die, der, mein, dein) it falls into a category known as "strong inflection". The adjective endings for such cases are:
ok even with going through the whole thing I found it hard! But I think I got it. It is because there is no article right? I mean, the "Das" in the beginning of the sentence is deceitful, but in the case after the verb, no article is specifying which or how many books...or?
But "Das" is not an article of Bücher so still I don't know why there is no "interesanten Bücher". I thought all adjectives of plural nouns should have an "en" at the end.
That's true in mixed inflection (e.g. after keine or ihre) and weak inflection (e.g. after die).
But here there is nothing before the adjective and so the adjective takes strong inflection.
Rule of thumb: in strong inflection, the last letter of the adjective is the last letter of what the definite article would be in that combination of gender, number, and case.
Bücher here is nominative plural and the definite article would be die which ends in E, so the adjective has to end in E as well (interessante) to show the gender, number and case -- since there is nothing in front of it which could do so.
Duolingo seems obsessed with marking 'they' wrong. I think these and they can be interchangeable in most situations.
So wait, Das sind zusaetzlichen Buecher gets an -en but Das sind interessante Buecher doesn't?
Nope, is should be "Das sind zusätzliche Bücher".
Bücher has no preceding article or determiner so that makes it a case of strong inflection. Because it is nominative and plural, the adjective takes the 'e' ending (see bottom table in link below):