"I am the adult."
Translation:Ich bin der Erwachsene.
Linking verbs (http://donnayoung.org/english/grammar/linking-verbs.htm) use nominative for both subject and predicate/object. Sein is a common German linking verb. Also werden, heißen, and scheinen.
It's derived from the verb "erwachsen," which in turn is the prefix "er-" on the verb "wachsen" ("to grow"). So "Erwachsene" simply literally means "grown-up [person]," much like the word "grown-up" in English for an adult. The "-e" at the end is an adjective ending, since "Erwachsene" is derived from an adjective (see below if interested).
That's probably about all you need to know, but if you want to get more detailed, the noun comes from the past participle of "erwachsen," which is still just "erwachsen." The noun is just that participle/adjective used as a noun, similar to how we might say things like "The land of the free and the home of the brave" in English ("free" and "brave" are adjectives but used as nouns here).
So since "Erwachsene" is derived from an adjective, it takes adjective endings. There's an "-e" ending for this sentence because we're using "der" (nominative case); just as you would use "-e" with "der" for a phrase like "der große Mann," you use the same ending as if we had "der erwachsene Mann" and thus "der Erwachsene." If the sentence were "Ich sehe den Erwachsenen," we would need the "-en" ending to go with the accusative "den" as if we were saying "den erwachsenen Mann"; similarly "Ich bin ein Erwachsener" for "ein" in the nominative. We would similarly use feminine endings/articles if the adult in question were known to be a woman.
This was probably quite a bit more information than you were wanting, but I hope it was helpful or interesting to you.
no wonder I hadn't a clue what's going on - this language does my head in - I passed B1 and have never ever come across all this before. I sometimes despair if I will ever understand German logic!!!! As to the linking verbs - never come across those either - a decade of VHS and private lessons and never been told these pearls of wisdom......
No, it isn't. It would be 'ein Erwachsener', however. Nominalized adjectives -although being nouns- are declined like attributive adjectives. http://en.wikipedia.org/wiki/German_declension#Attributive_adjectives You're looking for 'mixed inflection'. You may want to bookmark the page. It may come in handy quite often.
Sorry. I forgot that you don't see what my answer was.
I was asked to translate "I am the adult." I answered "Ich bin der Erwachsener" and was marked incorrect.
I see that erwachsen is "to arise", and thus yields "arisen/grown/mature" as an adjective, and thence "adult" in the noun form Erwachsene.
The point is: does not Erwachsener mean "adult male", and Erwachsene either "adult female" or "adults (plural)"?
You have to be careful here: nominalised adjectives are nouns but they are inflected as if they were adjectives. This means that the inflection not only depends on case, number and gender as it does for nouns but also on the preceding determiner. It's "der Erwachsene" (male adult, weak inflection) but "ein Erwachsener" (indefinite article -> mixed inflection)