Hello Jed,
There´s really not a reason why some words has exceptions, but I think most of it comes from dialects, old norse and so on. I tend to stick to the main rule, as I honestly don´t hear the difference between for example sykkel VS sykler.
Here´s some exceptions I found that are the most common:
den han vel men nok kan skal vil igjen
I would say that the "o" in tog is long, not short as you can drag the "o". Tooooog. If it had been short, it would sound like "togg".
You may have mixed dit (there) and ditt (yours). The "i" in ditt is short as the stress is on the double consonant -tt, the "i" in dit however is long since you can drag the "i" for as long as you like.
I hope this helped you out.
Best,
Ida
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