Looks like the 't' is silent, as is the 'h' in words beginning 'hv'. I'm new to Norwegian; using this basic stuff as a bit of a taster really. Must say I prefer my languages to be phonetic, especially in pronouncing consonants. German fits the bill but then the grammar is quite challenging. Still, can't have it all ways, I suppose.
So i've always thought that dutch would look simmilar to norweigian. how wrong i was. But Dutch also beeing a Germanic language, i can see some similarities and i can see some words that look and mean the same. I find it fascinating and i can't wait to speak this language. Greetings from the Netherlands
because the letter "r" from "er" its followed by letter "d" from "det". When "R" is followed by D,T,N,L,S it "ignore" the consonant, so the "det" become "et" in pronnounce. Another fact is that the "t" from "et" is silent, so the whole word "det" become a simply "eh" in pronnounce. "Er" + "eh" = "er'eh"
You must remember that the d does not become silent when you have r + d. Rather, the d is «colored» by the r, and both are replaced by a voiced retroflex stop. Similar cases are r+s, r+t and r+l. The sound example in this case is (for some reason) actually using a retroflex flap, which is used only in certain dialects in certain cases of r+d.
This video seems to be relevant (the person in it is saying the words with the Oslo-dialect). By the way, keep in mind that you will be understood completely fine without using retroflexes (and pronounce the R separately instead). We are used to hearing people with foreign accents. :) https://www.youtube.com/watch?v=TRegrgHDLq4
SupEvan is right (for almost all rolling R-dialects). But as a funfact: Some Norwegians actually use the ɾ in the words det, du, deg, de, dem after a vowel or an r. So «Hva er det» really could be pronounced /vɑː'æːɾə/. You should not really think too much about this unless you are a pronunciation geek, though.
The very simple answer is: yes, it is usually supposed to be rolled, and any Norwegian would understand you if you roll the «r» every time! ;)
The less simple answer is that it is often not rolled, for instance when it is in front of a «d» or «t», in which case the two sounds «melt» together and make a new sound, like here (although I would say that the Duolingo voice is a bit off).