"The chocolates are sweets."
Translation:Οι σοκολάτες είναι γλυκά.
No, because the English sentence is not "The chocolates are sweet" but "The chocolates are sweets".
That is, it is not telling you what the chocolates are like (are they sweet, sour, big, small, expensive....); it is telling you something about the category they are in (are they vegetables, fruit, sweets, ...).
So the translation should have a noun at the end, not an adjective.
In the singular (when there is just one thing), the definite article ("the" in English) is ο for masculine nouns, η for feminine nouns, and το for neuter nouns. (The gender of a noun can usually be guessed from ending, but it may be helpful simply to remember the gender for every noun.)
In the plural, the definite articles are οι for masculine or feminine, and τα for neuter.
Here, you have σοκολάτες which is plural (and feminine), so the definite article is οι. (Which sounds the same as η in modern Greek.)
Note that all these forms are for the nominative case, e.g. as the subject of a verb. In other cases, the articles can change.
Έναν is the masculine indefinite article 'a' in accusative or the cardinal number 'one'. Neither fits here. :)