He is sometimes breifly heard saying "Hell" and "Damn" in older videos
In his REALLY old videos (2012 or so), He cursed a lot and to this day, regrets that.
In a live stream video, DanTDM says "sh*t" and "f**k". Although this video wasn't posted on his channel.
In very old videos, Dan says the words "g*ddamn", "son of a b*tch", "c*nt", "f*ck" and "sh*t".
Dan now says "oh my God" a lot in new videos
Dan uses the words "Jackass" and "Oh my God" in the new episode "Addressing my Only Controversy"
Dan's older videos contained strong language, but after he restructured his channel to a family/child audience, he stopped with the swearing aside from his live streams. Nowadays, he does occasionally use at worst mild language, but nothing majorly bad.