"She went to the toilet."
Translation:Hun gikk på toalettet.
You can think of "gå på do" as a set phrase.
It's very uncommon to hear anyone use the definitive form unless they're referring to a specific toilet, such as the upstairs toilet (doen oppe) or the school toilet (skoledoen/doen på skolen). Or, occasionally, if they're going to the toilet to do something else than actually go to the toilet.
OK, but what if it's the only toilet in the house, or I happen to be pointing at the bathroom in question (i.e. each case making it a specific toilet), would you use "do" or "doen" in these cases? Also, what about when you precede "på do" with other verbs, such as, "er", "sitter", etc.?
If you are in the bathroom and on the toilet, and someone knocks on the door waiting to get in Jeg sitter på do lets them know that you are using the loo (as opposed to taking a shower). You could tell someone to sit on the toilet (on the lid), Sett deg på doen.
Asking directions Hvor er doen? Der er doen Where's the toilet? There's the toilet. Without på it's more natural to say doen.
Do is indeed a very common synonym for toilet Hun gikk på do would be the correct equivalent. Toalett is a very proper and clean term and I'd might prefer it in front of total strangers. Do is slightly more everyday speak, but it's still quite neutral. Dass quickly turns rude and offensive (if you want to yell at someone it's much easier to get enough 'mph with the double ss).
Mostly I will use do for the little room where I can go to the toilet, but doen would normally be the throne itself. Jeg må på do. Hun er på do. Har du vært på do? Har du vært på doen? Avisa ligger på do. Har du vaska do? Har du vaska doen? - I need to go to the toilet. She's on the toilet. Have you gone to the toilet? Have you been in the toilet room? The newspaper is in the toilet room. Did you wash the toilet x2.