"The brother of Sebastian is eight years old."
Translation:Fratele lui Sebastian are opt ani.
Why is "Fratele Sebastianului are opt ani" wrong? Female names seem to have no problem getting the -i suffix so can't male names just get the -ului suffix in the same way?
Female names ending in -a go to the -ei suffix in dative/genitive.
Ana -> Anei.
However, rare female names ending in consonant or male names just get the preposition "lui" which has nothing to do with the masculine possession pronoun.
Sebastian -> lui Sebastian.
I attempted to use "a" instead of "are" in the following response: "Fratele lui sebastian a opt ani"
I thought that "are" interchangeable with "a." Is that not the case?
That is not the case at all, as far as my knowledge goes. "are" is the third-person singular form of the verb "a avea" - "to have". So it is used when saying "he/she/it has". The small word "a", on the other hand, has a number of uses. It is generally an auxiliary verb or word with many uses.
For example, you'd use it to form feminine ordinal numbers: "a treia femeie" - "the third woman"
It is used to form feminine abolute possessive pronouns: "pisica asta este a mea" - "this cat is mine"
And it is also very often used to indicate the dative/genitive case.
However, I've never encountered "are" to be shortened to "a", I don't know if there is some dialect that does that. There are some similar pairs like "este" and "e" which are the same thing and interchangeable.