When do sarena and darien get together in sailor moon?

Answer 1

Serena and Darien first officially become a couple in the final Alan and Ann episode. In the DiC dub, that's episode 53 Treed. (Japanese: episode 59, The true love awakens! The Makaiju's secret)

However, two episodes later, Darien/Mamoru breaks up with Usagi/Serena!

BUT!! They get back together for good in DiC episode 70 Promises Fufilled (Japanese: episode 77, The Thoughts Are the Same! Usagi and Mamoru in Love Again)

At the end of the StarS anime, they become engaged.

However if you read the StarS manga (Act 52 first print, or Act 60 Japanese reprint) They get married, and at the wedding Usagi/Serena discovers that she is already pregnant with Chibiusa/Rini. Usagi keeps the last name of Tsukino.

In Naoko Takeuchi's last manga (A short titled Parallel Sailormoon) Usagi and Mamoru are still married and have had two daughters: Chibiusa/Rini and Kousagi, both of whom have their mother's maiden name of Tsukino.