U-shift (AKA u-hljóðvarp, AKA A-víxl, AKA u-umlaut (not to be confused with the German ü which confusingly goes by the same name), AKA u-mutation) is a sound shift that is triggered by a u appearing in a syllable immediately after an a in the stem of a word, where the a changes to an ö. If that sounded confusing, don’t worry: we’re here to make it easier!
U-shift is extremely systematic and happens all over the Icelandic language, but it is especially prominent in feminine nouns and verbs, which we’ll take a special look at here. These are by no means the only instances of it in the language, though, so don’t be surprised to see it elsewhere.
Basically, it works like this.
a before u changes to ö
Or, to put it more graphically,
a_u –> ö_u
A common mnemonic for this is: “a is allergic to u, so it hides behind ö”.
I prefer the slightly more vulgar mnemonic “a finds u disgusting so it goes behind the bushes to puke its guts out”. If you know how ö is pronounced, this makes sense. If you don’t, pay attention next time your buddy slams one too many vodka red bulls. You’ll learn some Icelandic pronunciation. Also you’ll learn that vodka red bulls are a bad idea.
Notice that u-shift only applies to a: NOT á or au. That’s because these are different letters representing different sounds.
When learning sound shifts like this, it’s important to remember that your conscious brain tends to be really bad at this sort of thing, but your subconscious mind is amazing at it (I promise it is, it’s a human thing).
Use this article as a reference, to reassure your conscious brain that a system exists, but don’t worry if you don’t get it right away. Read a lot of Icelandic, listen to a lot of Icelandic, try to notice u-shifts when you come across them, and before you know it you’ll be doing it automatically.
In the accusative and the plural, you add a u to feminine nouns. stelpa - stelpu, for instance. If there is an a in the stem (like kaka), this will trigger a u-shift.
Remember: your conscious brain doesn’t have to understand this perfectly. Just read a lot of examples, and your subconscious brain will get it.
In the first person plural (we), verbs add an -um in the present tense. að hugsa - við hugsum, for instance. Of course, if there is an a in the stem (like tala), this will trigger a u-shift.
Examples are important. Let’s take a look at a few.
There’s no u-shift in the masculine nominative, even though it frequently ends with -ur. This is why you can have word forms like svartur maður með kaktus, and not change the a.