Translation:I would like to speak to you about it.
I think that the two translations offered " would happily talk" and "would like to talk" convey in English two subtle different thoughts. "Happily talk" suggests a willingness, even an eagerness to talk. "Like to talk" suggests a need to discuss or desire to talk.
You're right, but both can be translated as "Ich würde gerne mit dir darüber sprechen". The difference lies in the intonation while speaking. For "happily talk" you would emphasize the "gerne" while otherwise you would not emphasize anything (the weight lies slightly on the "würde").
It's the past subjunctive.
For example, "Sie sagten, sie sprächen deutsch" = "They said they spoke German".
Also used for the conditional: "Sprächen sie deutsch, verstünden wir sie." = If they spoke German, we would understand them.
But in everday speech, Germans use "würde" + infinitive rather than the subjunctive: "Wenn sie deutsch sprechen würden, würden wir sie verstehen."
They're slightly different.
"Ich würde gerne mit dir sprechen" = "I would like to speak with you".
"Ich möchte gerne mit dir sprechen" = "I would like to speak with you." / "I want to speak with you".
In English, they're the same, but in German, the first is slightly more distant or polite.
In English, "speaking with" and "speaking to" can have quite different implications. The latter has a preachy feel; a one way conversation. Can a similar distinction be made in German? Duo lingo seems to preference "speak to" as the translation for "sprechen mit". Perhaps "sprechen an" is possible?