"He is not your rival."
Translation:Él no es tu rival.
I am sure that you now understand the reason for using ser rather than estar, but for those out there who have yet to understand, please allow be to explain:
The verb ser is used to express description, profession, point of origin, identification, material, possession or ownership, and where an event takes place. Estar is used to describe location, health, changing mood or condition, and personal opinion in terms of taste or appearance.
In this sentence the conjugated form of ser (es) is used to describe “El” as not being your “rival”.
Why would "él no es su rival" be incorrect? I thought that su was the informal of "your"?
Your phrase is correct, it is a Duo error, but "su" is not the informal form, it is the formal one.
I reported it. It should be counted (last question, now I have to start over). But yea su is formal, connected with usted.
Su is formal "your" but also "his" and "hers" and "its". Tu is less ambiguous here.
right,... su rival ... could also mean your pl : more than one person who thinks 'he' is their rival... right?