Whether undir takes accusative or dative, depends on the sense.
- With the accusative case, it signifies movement towards something or someone. In other words it has an allative sense.
- With the dative case, it signifies positioning and location somewhere. In other words it has a locative sense.
- undir in Geir T. Zoëga (1910) A Concise Dictionary of Old Icelandic, Oxford: Clarendon Press