Differences between "vuestros" and "sus"
As a beginner in learning Spanish, I'm really CONFUSED with the use of "vuestros" and "sus". PLEASE HELP!!!
Please see the following examples.
1 Abuela: Marisa y Clarisa, ;qué bonitas son____ faldas! ¿Son nueva?
2 DORA: ¿_____hermanos no están casados?
ANA: Mis hermana es soltera.
My answer for both blanks is "vuestors", but the correct answer is "sus".
Why is that? Can someone Please explain it to me and help me to know how to use these two correctly?
Thank you Very Much.
The Sad Lady
5 Answers
Like Stadt says, "vuestros" and "sus" are equivalent in number and gender, son unless there's anything in those sentences that tells you whether you are talking in the more polite "ustedes" or the more informal "vosotros", both can be correct. Still, it is probably not a good idea to confuse informal with slang, "tú" and "vosotros" can be used in formal conversations as long as there is enough trust or familiarity between the speakers, depending on the country. In other words, assimilating "tú" with "you all" is taking it a little too far 
One thing that is probably very confusing about usted/ustedes is that the pronouns and conjugations used for them are identical to the pronouns and conjugations used for él/ellos, respectively. That's why saying "sus faldas" can mean either "your skirts" or "their skirts". In your case, it seems that grandma is looking at Marisa's and Clarisa's skirts, and is telling them: "how beautiful are your skirts!". The answer would be "vuestros" or "sus", and you can't tell which one from the context.
In the second case it's more of the same. The question is: "aren't your brothers and sisters married? And Ana's answer (although with a typo) is: "My sisters are single". So again, the right answer is either "vuestros" or "sus", and you can't say which one is correct.
One guess is that the book you got that from was written by a Spanish speaker that uses "usted/ustedes" for the majority of cases.
To answer Billstpor's comment, I took this from the site above:
When you are showing possession with compounded nouns, the apostrophe's placement depends on whether the nouns are acting separately or together.
Miguel's and Cecilia's new cars are in the parking lot. This means that each of them has at least one new car and that their ownership is a separate matter. Miguel and Cecilia's new cars are in the parking lot. This construction tells us that Miguel and Cecilia share ownership of these cars. The possessive (indicated by 's) belongs to the entire phrase, not just to Cecilia. Another example:
Lewis and Clark's expectations were very much the same. This construction tells us that the two gentlemen held one set of expectations in common. Lewis's and Clark's expectations were altogether different. This means that the expectations of the two men were different (rather obvious from what the sentence says, too). We signify separate ownership by writing both of the compounded proper nouns in the possessive form.
So your usage was correct, as it was each of their skirts, and not a set of shared skirts.
"Vuestro(a)" is your (plural, informal- you all's in improper English). "Su" is their. Also his, her, or your (formal- plural or singular).
In the first conversation it appears that they are talking about other people, so you would use "their".
In the second sentence I don't see enough context to be sure.
In the final sentence, she is speaking of her sister, so she would use my="mi". I am not sure why mis is there.
It is difficult to be sure without a little more context- because the more I look at these sentences the more they seem open to multiple interpretations as to who is speaking to who about whom.
If you were using "vuestro" before "faldas" you would use "vuestras" because you need plural and gender agreement.
Vuestro/s is more specific that su/s. When you hear vuestro/s someone is being adressed, just like when you hear tú, tu, or usted, ustedes. Su and sus can refer to a 3rd party.
Stadt and Billstpor, thank you very much on answering my question. Yes, there is a typo on Ana's answer. It should be "Mi" instead of "Mis". Again, thank you for correcting that. 
