There is not article ('een') in the dutch sentence, putting it in will change the meaning a bit. Learning to translate means to pay attention to details, and at the same time it has to be idiomatic English. The idiomatic differs sometimes, if not keep the original meaning. Translating is dancing on a fine line.
You won't say "a water", but you absolutely can say "a soup" meaning "a plate of soup", "one soup". In my opinion both "a soup" and "soup" should be accepted in this case.
- "de soep" (the soup) - "het voorgerecht" (the appetizer)
- "Jij eet de warme soep" - "Jij eet het warme voorgerecht"
- "Jij eet een warme soep" - "Jij eet een warm voorgerecht"
- "Jij eet warme soep" - "Jij eet warm voorgerecht"
The adjective gets an -e when the noun is masculine or feminine, or when it has an article. / The adjective doesn't get an -e when the noun is neuter and there's no definite article. (Not an ideal combination, but it's rare to have meaningful sentences for each of definite article, indefinite article and article-less.)
The original word is "jij". Hence, you can always use that. But if the word is not important, you can use the eroded form: "je". The most common situation where you can't use the eroded form is when there's stress on the word "jij". - "Zíj zijn groot en jíj bent klein." (They are big and you are small. - Stressed for the comparison.) - "Jíj hebt het gedaan!" (You did it! - Stressed to indicate it's not someone else.)
Note: "Je" is also the eroded forms of "jou" and of "jouw". Plus, it can be used as an alternative for "men".
But it isn't. If you're ever offered to define your own Dutch, you can decide to remove all legacy you don't like. If you're merely trying to use the same Dutch that the Dutch use, you can't use the eroded forms where the full forms are required. Of course, feel free to report cases where the full form is not accepted, for some reason.
You can always use the full forms (jĳ, gĳ, zĳ, wĳ, mĳ, jou), but you can only use the eroded forms when those are unimportant. When using a comparison, use "jĳ" etc, when combining a pronoun with a different subject or object, do so as well. When stressing a pronoun, use stress on a full form (jíj, gíj, zíj, wíj, míj or jóú). But I hope there's a separate lesson about stress.