There first date is on the season 3 finale episode called "The Job" but they are confirmed as boyfriend and girlfriend on the season 4 premiere episode "Fun Run" part II
The episode is called "gay witch hunt"
Season 5, Episode 2: Business Ethics
Product Recall. Season 3, Episode 20.The episode actually starts with Jim impersonating Dwight.Then, vice versa at the end of the episode.
Lovefool- the kardigans
Season 3, Episode 18: The Negotiation.
Season 4, Episode 13 the name of it is Job Fair
The very first episode, Season 1 "Pilot"
season 3 episode 13 half way through the episode
Season 4, Episode 10 (Series Episode 63): Branch Wars
season 3 episode 20 "product recall"
Niagara: part 2 ! It was awesome
Season 3 "Business School"