"The elephants are yours."
Translation:Gli elefanti sono i tuoi.
There are any number of reasons why it could have marked you wrong (glitch, typo, it doesn't think you should know it yet...) But yeah, English as it is taught doesn't distinguish between singular and plural "you" and we don't have a formal vs informal. So without any clarifying context, "you" could refer to one person or to many. But you had the "i vostri" agreeing with the elephants correctly, so strictly speaking that was also a valid translation.
It only elides in the singular: l'elefante. The plural doesn't elide: gli elefanti.
tuo = your (singular masculine)
tuoi = your (plural masculine)
tua = your (singular feminine)
tue = your (plural feminine)
suo = his/her (singular masculine)
suoi = his/her (plural masculine)
sua = his/her (singular feminine)
sue = his/her (plural feminine)
These must agree with what is owned, not who owns it.
In this case tuoi is predicate, so it has to be the same gender and number of the subject (gli elefanti); in Italian it's the same as if it were an adjective, i.e. "gli elefanti sono tuoi" (the elephants are yours) and "sono i tuoi elefanti" (they're your elephants) differ only in article and order.
"La tua" is for a singular feminine thing that you have. "Gli elefanti" is plural masculine.
Possessive adjectives and pronouns work just like any other adjectives. They must agree in gender and number with the thing that is possessed, which is reflected in the end of the word. The first part of the word says broadly whose it is.
il mio (my singular masculine thing)
i miei (my plural masculine things)
la mia (my singular feminine thing)
le mie (my plural feminine things)
It does not matter who "I" am.
TU (singular "you", addressing exactly one person)
il tuo (your singular masculine thing)
i tuoi (your plural masculine things)
la tua (your singular feminine thing)
le tue (your plural feminine things)
It does not matter who "you" are.
il suo (his/her singular masculine thing)
i suoi (his/her plural masculine things)
la sua (his/her singular feminine thing)
le sue (his/her plural feminine things)
Again, the gender of the possessive reflects the gender of the thing, not whose it is.
il nostro (our singular masculine thing)
i nostri (our plural masculine things)
la nostra (our singular feminine thing)
le nostre (our plural feminine things)
VOI (plural "you/y'all", addressing two or more people)
il vostro (y'all's singular masculine thing)
i vostri (y'all's plural masculine things)
la vostra (y'all's singular feminine thing)
le vostre (y'all's plural feminine things)
il loro (their singular masculine thing)
i loro (their plural masculine things)
la loro (their singular feminine thing)
le loro (their plural feminine things)
"Loro" is the exception here in that the possessive form does not change, but the article still shows agreement.
English "a" and "an" are not interchangeable even though they are both indefinite articles. Italian "i" and "gli" are not interchangeable even though they are both plural masculine definite articles.
Yes. The possessives must always agree in gender and number with the noun they modify, never with whose it is.
Yes, that would also be correct.
"Gli elefanti sono i tuoi" means "The ELEPHANTS (and not something else) are yours."
"Gli elefanti sono tuoi" means "The elephants are YOURS (and not someone else's)."
However, it must always be "i tuoi elefanti" because the possessive adjective requires the definite article (except with singular unmodified family members).