They dance in the April Fool's episode where Helga is pretending to be blind. I don't know if they dance in any other episodes though :)
Helga never died of anything.
It was either Gerald Comes Over or Helga Blabs It All.
Hey Arnold - 1996 Arnold Betrays Iggy Helga and the Nanny 3-10 was released on: USA: 19 October 1998
Because that is how they wanted her character to be.
Hey Arnold - 1996 Arnold's Room Helga vs- Big Patty 3-4 was released on: USA: 9 September 1998
Hey Arnold - 1996 Helga's Show The Flood 3-15 was released on: USA: 25 November 1998
Hey Arnold - 1996 Helga's Locket Sid and Germs 5-7 was released on: USA: 19 April 2000
Hey Arnold - 1996 Helga Sleepwalks Fighting Families 4-13 was released on: USA: 23 October 1999
Hey Arnold - 1996 Helga's Masquerade Mr- Green Runs 5-2 was released on: USA: 11 March 2000
Hey Arnold - 1996 Helga's Love Potion Gerald's Secret 2-2 was released on: USA: 28 September 1997
Hey Arnold was a cartoon that aired on Nickelodeon from 1996 until 2004. The episode The Journal was a special one hour episode that aired on November 11, 2002.
Try looking on google :). type the name and what episode it is.