Think of the fourteen grammatical slots as three baskets.
● First group: five slots whose final root letter carries ḍammah and which have no inflectional nūn, such as يَفْعَلُ.
● Second group: seven slots represented by five written forms containing the final inflectional nūn, such as يَفْعَلَانِ and يَفْعَلُونَ.
● Third group: two feminine-plural slots ending in نَ: يَفْعَلْنَ and تَفْعَلْنَ.
Fourteen slots do not mean fourteen different spellings: context and the pronoun can distinguish repeated forms.