There's potential for a lot of confusion between different conjugations of indicative, subjunctive and imperative: e.g. "llama" is el/ella/ud. indicative + tu imperative; llame is yo/el/ella/ud. subjunctive + ud. imperative.
Anyone have any guidance on how to distinguish between them all? Just use exclamation marks and watch out for "doubtful" clauses indicating subjunctive? Any tips from native speakers...?
Not a native speaker, but it's all context. Not a reassuring answer, but if someone runs at you yelling, "Llama una ambulancia," it won't be ambiguous.
I'm sure if we thought about it, we could find a good number of ambiguities in English like homophones, homonyms, and the like that confuse learners while never bothering native speakers. (And how do we even survive with only one form of second person pronoun for both singular and plural.)
As we learn, it--hopefully--will get much easier.
When "llamar" means to inform, notify, or warn, then "la ambulancia" is considered a direct object, which needs the "a" preposition.
When the verb "llamar" means to phone or call someone, it has an indirect object, which also needs the "a" preposition. For example: "La (direct object) llamé a Marta" (Marta= indirect object), or you can simply say: "Llamé a Marta". This last sentence is more accurate in the gramatical sense, since "La" can be considered a redundancy. Anyway is more common to hear people saying: "La llamé a Marta", at least here in Argentina.
When "llamar" means to name or qualify someone or something, then the "a" preposition is not necessary. For example: "La llamé Ana" (I called her Ana) (for example from a father when his daughter was born), or "Lo llamé anticuado" (I called him antiquated).
In conclusion, I think Duolingo is wrong in this case, or at least should consider "Llama a una ambulancia" a correct answer. Some people can avoid the "a", maybe in some countries more than here, but I think Duolingo should restrict to correct grammar more than common usage of words, almost impossible to learn due to their enormous quantity. Anyway, common usage of words can be considered incorrect at the grammatical level, but it usually imposes itself with time until it becomes part of what then is considered correct, so... time will tell.
Thank you both! I included "a" and was marked wrong. I will report it. I was wondering if the absence of the "a" in the "correct" answer had anything to do with the fact that an ambulance is a thing rather than a person? Or is it just that the preposition "a" is omitted sometimes in contemporary usage? I'd be curious to hear from other native Spanish speakers in other countries how you would say "Call an ambulance!"
The "personal a" is used in direct objects that are people (well, and pets), but not for an "item" like an ambulance. http://spanish.about.com/cs/grammar/a/personal_a.htm
With llamar, what i have learned is that you aren't actually calling the item ambulance, you are calling people responsible for the ambulance, thus is still requires the personal 'a'. Same thing as "llama al hospital", "llama a la escuela". You are calling the people in those locations