"Your friend is my enemy."

Translation: Dein Freund ist mein Feind.

[E5Pepe]

"Deine Freundin ist mein Feind."

Why it is not accepted?

[mizinamo]

Deine Freundin ist meine Feindin is accepted.

If you're going to be specific about your friend being female, the list of alternatives expects you to specify "female enemy" as well.

[RikNijhuis]

I would say that Gegner should work as well?

[Arisened]

i think gegner is more like opponent (as in sports)

[deactivated user]

Der Freund meines Feinds ist mein Feind.

[freemanjw]

Don't quite understand why Feind is not accusative case here

[mizinamo]

"to be" is not a transitive verb that takes a direct object.

You can't say that the enemy "is being been" by your friend.

"to be" is a copula - a linking verb that links a subject to a predicate that says something about the subject.

Such predicates are (almost always) in the nominative case in English. (And, traditionally, in English -- conservative speakers will say "it is I" and not "it's me".)

[MattAshton3]

What case is this, and why is it Ihr?

[mizinamo]

dein Freund and mein Feind are both in the nominative case.

If you saw Ihr Freund -- that's the form used when speaking politely to one or more people; Ihr (always capitalised) is the possessive corresponding to Sie (always capitalised), the polite "you".

[beatinbuddy]

I don't understand why Gegner isn't usable here with they both mean approximately the same thing.

[Arisened]

i think gegner is more like opponent (as in sports)

[beatinbuddy]

The sentence "I have an enemy" accepted "Ich habe einen Gegner".

[dioguo]

"Eure Freund ist mein Feind." wasn't accepted. Isn't "Eure Freund" the correct way of saying "Your friend" adressing two or more people?

[mizinamo]

Isn't "Eure Freund" the correct way of saying "Your friend" adressing two or more people?

No.

Freund is masculine, but eure is the form used before a feminine or a plural noun. (Mnemonic: *die ends in e.)

It would have to be euer Freund, with no ending at all after the r, as in ein Freund.