I probably should have mentioned a few more details in my first answer (in order to confuse you even more...):
In more informal settings, in which neither of the people involved is a professional teacher, and you just want to say that one of them is casually teaching the other person how to do something (no structured class / no professional teaching skills required), we can use the term "å lære noen noe" instead of "å undervise". In that case, the person who has a skill or some knowledge about something "lærer bort noe til en annen" (literal translation: teaches away something to somebody else).
Some examples:
- Han ville lære datteren sin å sykle
- Hun lærer kjæresten sin å spille tennis.
- Han lærte lillesøsteren sin å knyte skolissene.
- De bestemte seg for å gjøre en språkutveksling: Han skulle lære henne å snakke polsk, mens hun skulle lære ham å snakke norsk.
- De to guttene hjalp hverandre med leksene: Per lærte Knut hvordan man bøyer tyske verb, mens Knut lærte Per å regne ut areal og omkrets av en sirkel.
Let me know if any of this was unclear, and I´ll try to explain in further detail.