"I saw you once at my uncle's house."
Translation:Te vi una vez en la casa de mi tío.
Yes the difference is subtle but it's there. When you have once at the end, the sentence sounds complete. If you mix the two, "I saw you there once, I tried to call you," it sounds very awkward.
At the beginning, "once" begs for another clause. For e.g., "I tried to call you once I saw you there." Instead of indicating a number here, "once" at the front carries a meaning close to "as soon as."
It says the following sentences are both correct: 1 - Los vi a ustedes una vez en la casa de mi tío. 2 - Te vi una vez en la casa de mi tío.
My question is - why does the the first translation have a personal "a", whereas the second translation doesn't? Could the first one also be "Ustedes vi una vez en la casa de mi tío" and could the second one be "Lo vi a ti una vez en la casa de mi tío" ?
Think of the first translation as "Los vi (a ustedes) una vez..." i.e. "los" is the mandatory direct object pronoun (which could mean them or you plural) and "a ustedes" is the optional redundant pronoun phrase that clarifies that "los" here means "you" (plural). Hence, it can't be "Ustedes vi una vez..." because "ustedes" (a subject pronoun) is not a direct object pronoun, "los/las" is. Note: Do not think of this as using a "personal a." All the Spanish articles I've gone through refer to it as a prepositional pronoun phrase.
In the second translation, "te" is not ambiguous like "los" above. It always means "you" (familiar) and so "a ti" isn't required at all as there's no confusion there. However, if you add it nonetheless, it's not incorrect and would just mean an added emphasis on "you." For example, if you said you saw me at.. but I reply, "No, I wasn't there," you could say, "No, I saw youuuuuu at..." That's what "a ti" would do.