o ... ö :-- koma (to come), kömr (comes).
ō ... œ :-- fōr (went), fœra (bring).
au ... ey :-- lauss (loose), leysa (loosen).
jū (jō) ... ȳ :-- sjūkr (sick), sȳki (sickness); ljōsta (strike), lȳstr (strikes).
23. The change of a into ę is sometimes the result of a following k, g, or ng, as in dęgi dat. sg. of dagr (day), tękinn (taken), gęnginn (gone), inf. taka, ganga. i appears instead of e, and u instead of o before a nasal followed by another cons.: cp. binda (to bind), bundinn (bound) with bresta (burst) ptc. prt. brostinn.
24. There is also a u-mutation, caused by a following u, which has often been dropped:
a ... ǫ :-- dagr (day) dat. pl. dǫgum; land (land) pl. lǫnd.
25. Unaccented ǫ becomes u, as in sumur pl. of sumar (summer), kǫlluðu (they called), infin. kalla.
Fracture
26. The only vowel that is affected by fracture is e: when followed by original a it becomes ja, when followed by original u it becomes jǫ, as in jarðar gen. of jǫrð (earth)4. When followed by original i, the e is, of course, mutated to i, as in skildir plur. nom. of skjǫldr (shield), gen. skjaldar.