In Devarim, 13:12, the text reads "יִשְׁמְע֖וּ וְיִֽרָא֑וּן וְלֹֽא־יוֹסִ֣פוּ לַעֲשׂ֗וֹת " which is translated (on Sefaria) as "will hear and be afraid, and such evil things will not be done again". The Stone translation is "shall hear and fear, and they shall not again do such an evil thing". [The former has yosifu translated into the passive and the latter, in the active but that's a question for another day.]
But no one I have found seems to explain why the middle verb, yira'un has a final nun appended to it. In Shoftim, (19:20) the text reads " יִשְׁמְע֣וּ וְיִרָ֑אוּ וְלֹֽא־יֹסִ֨פוּ לַעֲשׂ֜וֹת ע֗וֹד" which the Sefaria/JPS translates as " will hear and be afraid, and such evil things will not again be done". The loss of the final nun (turning the middle verb into yira'u) does not seem to have affected the meaning.
Shoftim 17:13 has another set of 3 verbs and the third (y'zidun) is the only one with a terminal nun. Yosifun appears only once in Tanach (Melachim 1, 19:2)
What does a final nun do to a verb which would explain its use or non-use? Are there any commentaries which place any explicatory significance on the use or non-use of the nun?