The medio-passive forms are formed by adding the reflexive or personal pronoun for the respective persons. In the 1st person singular the pronoun mik 'me' was added. This happened in pre-ON. times, when the 1st person ending was -u or -ô in most paradigms. When mik was added to the verb the form kallômik 'I call myself' appeared. In ON. this became köllumk. In the same way the pre-ON. 3rd person form kallôR-sik became kallask in ON. Originally sik was used only with verbs that had its object in the accusative, whereas the dative form sér was used with those with the object in the dative. Adding sér gives us ON. forms like 1st pers. sing. köllum and 3rd pers. kallas. The distribution between acc. and dat. forms does not remain.

Through quite a few analogical changes paradigms like these appeared:

	the weak verb kallask:						the strong verb þrífask:
		present			preterite			present			preterite
sing.	1	köllumk		1	kölluðumk		1	þrífumk		1	þrifumk
	2	kallask		2	kallaðisk		2	þrífsk		2	þreifzk
	3	kallask		3	kallaðisk		3	þrífsk		3	þreifsk
plur.	1	köllumsk		1	kölluðumsk		1	þrífumsk		1	þrifumsk
	2	kallizk		2	kölluðuzk		2	þrífizk		2	þrifuzk
	3	kallask		3	kölluðusk		3	þrífask		3	þrifusk

In the 1st person singular the ending is -umk. It causes u-umlaut, and in the preterite of strong verbs it's added to the plural stem(!). That's why it's þrifumk and not **þreifumk.

The rules for making the other persons medio-passive are quite simple. When -sk is added to an active ending
• a /r/ disappears (kallar + -sk = kallask)
• a /ð/ or /d/ turns to /t/ (kallið + -sk = kallizk (/ts/ is written z))
• a /t/ is inserted after /ll/ and /nn/ (finnr + -sk = finzk)

These endings are the ones found in the oldest OIcel. texts. ...
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