"I am eating an apple."
Translation:Ich esse einen Apfel.
Because Apfel in is accusative case in this sentence. I eat what? - an apple. Masculine indefinite article for accusative is einen. Here's more on cases and an article on declension:
Hope that helps.
If you are using the mobile app, these options won't appear. However, if you use the desktop site then you can see that Duolingo offers information and examples before you start answering questions. I suggest going through a couple levels each lesson on the desktop site before using the mobile app to finish them.
You have to learn it. If they ask you to translate an English word to German and you don't know the gender, look it up in a dictionary. And when you learn a new word in German, don't just learn the word, you must learn the gender. The best way to do this is to learn the nominative definite article that goes with the noun (der, die, das). So for apple (if this is your first time learning the word), don't learn "Apfel", learn "der Apfel". A feminine noun like Forelle (trout) learn "die Forelle". A neuter noun like Brot (bread), learn "das Brot". There are some general rules to help you guess or remember some genders. If you search in the discussions or maybe even Google you can find some hints and tips.
Hope that helps, best of luck!
German (and many other languages) do more conjugating than English. "I eat" but "she eats". "Ich esse" aber "sie isst". Here is the conjugation table for "essen": http://www.canoo.net/inflection/essen:V:haben
For now, just focus on the first column on the left (present - indicative) and ignore for now the forms in that column that use an eszett "ß". You may come across those forms in old books, but modern spelling for those words (in this case, not all cases) is the double "s"...esst and isst.
Don't get overwhelmed looking at the tables, just focus on the one I pointed out for now. Hope that helps!
Click on the "cases overview" link, then go back to the original link, click on "nominative case", then do the same for accusative (and the other cases if you're interested, but maybe wait until you get a handle on the accusative).
Short version, nominative case is the subject, accusative is the direct object.
I am eating an apple. I am eating = nominative, an apple = accusative. The accusative answers the question "what?". I am eating what? An apple. An apple is accusative.
When you read the accusative link at the website I am sending you to up there ^ pay attention to the verbs that are followed by nominative. These will trip you up later on.
Hope that helps!
If you can ask "wen oder was....(who or what... do you eat)" then it's accusative, if you can ask "mit wem oder womit... (with whom or with what.... do you play)", then it is dativ. Was ist du? Ich esse eineN Apfel. Womit spielst du? Mit eineM Apfel. Very difficult even for many German speaking people;)
http://german.about.com/library/weekly/aa042098.htm is a good summary. When I learnt German though, nobody told me (until much later in my studies, at which point, I knew them anyway) about a GREAT tip to not have to remember the gender of many words ending in a similar fashion... I explain badly, but basically, here is an explanatory table: http://www.lsa.umich.edu/german/hmr/Grammatik/Gender/Gender.html (I particularly like the end of this article where it mentions how articles influence how you 'feel' about words. I'm a native French speaker and definitely found it hard to switch to the German way of thinking -for me, a table is definitely feminine, but the German der Tisch made me switch my brain into thinking of it as masculine!) :)