"I am your partner."
Translation:Ich bin dein Partner.
When the verb of the sentence is 'sein/to be', the nouns on either side of the verb are treated as the subject/nominative. This is also a rule in English; if you were to knock on a friend's door, and your friend asked, "Who is it," you might respond with, "It's me," but the "correct" way to put it is, "It is I."
My guess behind the rule is that, since you're saying that something is something else, you are essentially describing them as the same thing, and so they should be treated as the same part of speech.
Also, with a masculine noun in the accusative, the article/possessive pronoun ending changes to -en, not -e. For example: "I see your partner." = "Ich sehe deinen Partner." I hope this helps.
You're not alone! Many English speakers have trouble with "you" and "your" in languages like German. In English we use "you" and "your" to refer to a single person or several people, but in German "dein" = "your" singular and "euer" = "your" plural. Both should be accepted here. "Eure" would only work if the original sentence had been "We are your partnerS." because it implies plural both on the side of the "owner" as well as the "owned".