For this emphatic construction, an emphatic lām comes at the beginning of the imperfect and an emphatic nūn is added at the end.
● Light nūn: نْ — sākin
● Heavy nūn: نَّ — mushaddad
The heavy nūn is used more often, and this construction is a strongly emphatic style found frequently in the Qur'an. Do not turn that statement into fixed lexical meanings such as “light = moderate” and “heavy = extreme.”