"I do not want to be your friend."
Translation:Ich will nicht dein Freund sein.
That's poetic sentence order -- sort of like "Thy friend I shall not be" might feel in English.
Technically correct, perhaps, but not the level of language this course is aimed at, so in effect "wrong" if your goal is to speak naturally.
Maybe it has something to do with "to be"?? Because earlier in this lesson we had this sentence: Man kann diese Tassen nicht ersetzen. There nicht comes near the end.
No, it is not.
Both sides of "to be" (including "not to be") are in the nominative case.
"dein Freund" is a predicative noun with possessive pronouns. To be something comes in Nominativ therefore dein. Deiner would be either Genitiv or Dativ.
The possessive adjectives (mein, dein, unser, etc.) inflect like ein or kein -- so they have no ending in the masculine nominative, or in the neuter nominative/accusative.
In another example (Wir wollen deine Mutter nicht sauer machen) the comments said that putting "nicht" before "deine Mutter" would mean "we would not want to make (specifically) your mother angry". So why does this one not mean "I do not want to be your friend (but I might consider someone else)"?
So why does this one not mean "I do not want to be your friend (but I might consider someone else)"?
It can mean that.
It's about as ambiguous as the English "I do not want to be your friend" -- depending on which word you emphasise, it could mean "I do not WANT to be your friend" or "I do not want to be YOUR friend" or "I do not want to be your FRIEND" or "I do not want to be your friend" or just the unmarked "I do not want to be your friend".