When I'm using the imperative mood, does the subject come before of after the verb? I ask because in "Viva Las Vegas" the subject comes after- if this is indeed the common form of imperatives, why is the usual SV ordering abandoned?
Technically, phrases such as Viva Guatemala are not imperative, but subjunctive.
Specifically, they are the subjuntivo exhortativo (u optativo). Technically there's nothing preventing them from having the subject in front, but there are two reasons that may cause the more common VOS/VSO from SOV/SVO.
- In modern times they are used almost as if (and to some, simply as) imperatives thanks largely to uses with usted and nosotros/as (Gramática 42.4f) , and the true imperative has a strong VOS/VSO order, even rejecting pre-verbal complements.
- It is more common these days to explicitly subordinate these sentences with an implied principal clause by using an initial que. Subordinate clauses tend to prefer (but don't require) VOS/VSO ordering unless the subject is contrastive.
Because «¡Viva Las vegas!» is among cases of imperative structures which express good fortune, applause or enthusiastic homage to something or someone. For this to happen, the 3rd person of the present subjunctive of the verb 'vivir' must be followed by a noun. As such it can't be said is just a common form of imperatives. Other examples are:
«¡vivan los novios!», «¡Vivan las Cortes de Cádiz, viva la desamortización!» or simply «¡viva!».
Don't take my word for this, but I believe in cases like this it's not necessary imperative, but subjunctive and might be translated something like "May Las Vegas live!" I say this because I've heard things like, and forgive the vulgarity, "¡Que te folle un pez!" translated as "May you be fucked by a fish!"
You have been told this is not an imperative sentence. But, let's talk about imperative. There are still cases on which you can state the subject of the command if you want. These require a plural imperative:
¡Váyanse ustedes dos a comprar las tortillas! (emphasis on you both)
¡Cómprense los dos un dulce! (one piece each)
¡Corran todos los niños! (just the boys, but emphasis on ruuuuun)
¡Lavemos tú y yo los platos! (since no one else is doing it)
You can still use an apposition:
¡Váyanse, ustedes dos, a comprar las tortillas! (this would mean there are more than two children, so you are choosing)
Singular form requires apposition: Maria, ven. You may not require it always, for starters any vocative: Oye abuelo, // Oigan niñas, // ¡Sal tío, te estamos esperando! // ¡Váyase abuelo!, no lo detengo.
As for the order, you can use SVO with "Todos" "¡Todos corran por sus vidas!" (emphasis on who)
And you MUST use it with "Nadie" "¡Nadie me siga!" (no one, absolutely no one) alternatively you can leave it at the end, but you must add "No": "No me siga nadie" (I do not want to be followed)
As to why this natural VOS order (¡vamos + al cine + tío!) order, I can just say there must be some psychological factors in place that would require to be analyzed, if they have not been analyzed yet. But I would guess it is the importance on what is to be done, rather than who is to be doing it.
As for Viva sentences, I always think of them as prayers or wishes. (May) God save the Queen! May Las Vegas live. They are not commands or imperative sentences, so they should not be compared.