Mudari Takhfif is recognized through a change at the verb ending.
Group
ordinary imperfect
Mudari Takhfif
change
First
يَفْعَلُ
يَفْعَلَ
final-root dammah becomes fatḥah
Second
يَفْعَلُونَ
يَفْعَلُوا
inflectional nūn is removed
Third
يَفْعَلْنَ
يَفْعَلْنَ
no visible ending change
The particle comes before the verb, but the visible change appears at the end of the verb.