olivas is accusative plural and is used often as the direct object for verbs, the thing having the verb done to them. nos olivas habemus (we have the olives).
olivae can be the nominative (and vocative) plural which is the subject, the thing 'doing' the verb. olivae nos habent (the olives have us). It can also be the genitive or dative singular.
As to English, it feels to me more usual to indicate an indefinite article with plurals or uncountables; eg some olives, some bread. Obviously not if you are answering a choice question, "Olives or pickled onions?" I have tried using "some", but unlike "the", which isn't there in classical Latin, DL hasn't accepted it when I did so.
MagistraKate is right on that we do not have firm evidence that classical Latin used the subjunctive on its own in this way, unless DL implies an ellipsis. Some modern languages, such as Spanish and German, can address the tone of a request in part based on using the formal or informal personal pronoun. In Spanish, you can use the imperfect subjunctive for a polite request, Quisiera unas aceitunas. But you can also just say quiero unas aceitunas, por favor. Current evidence is lacking that classical Latin used subjunctive in the way DL currently teaches with these short velim sentences. Romans tended to use the imperative or indicative and could soften the request with words such as obsecro, quaeso, amabo + ut + subj. Here's a link: https://www.latinitium.com/blog/politeness-in-latin Language is culture, and one of the key ways ancient Romans communicated to each other was by the person's dress. If you were wearing a toga, you were addressed differently than if you did not have a toga. If your toga had a purple strip, then you would expect greater deference when being addressed. See this link https://www.thoughtco.com/six-types-of-toga-in-ancient-rome-117805 Whether the servers at the tavernae were plebeii or servi, they were almost certainly not elite Romans. If anyone can bring forth evidence from ancient texts or papyri for the use of the subjunctive on its own in this way, please do so. Cicero to Atticus 9.19.4 scribas velim, "I wish you would write," (Leumann-Hofmann-Szantyr, Lateinische Grammatik, p. 330) is an example of an optative subjunctive in the form of a wish but it's not a general request.