Apply the same three-group map used for Mudari Takhfif.
Group
Ordinary imperfect
After أَنْ
Change
First
يَكْتُبُ
أَنْ يَكْتُبَ
final dammah becomes fatḥah
Second
يَكْتُبُونَ
أَنْ يَكْتُبُوا
inflectional nūn is removed
Third
يَكْتُبْنَ
أَنْ يَكْتُبْنَ
no visible ending change
The particle comes before the verb; the visible grammatical change appears at the verb ending.