In يَفْعَلُونَ and تَفْعَلُونَ, the wāw signals masculine plural and the nūn is the indicative inflectional ending. Before the emphatic nūn, both the inflectional nūn and the plural wāw disappear, but the ḍamma before the emphatic ending preserves the plural distinction.
يَفْعَلُونَ ← لَيَفْعَلُنَّ — the men will certainly do.
تَفْعَلُونَ ← لَتَفْعَلُنَّ — you men will certainly do.
Compare singular لَيَفْعَلَنَّ: the singular has fatḥa before the nūn, while the masculine plural has ḍamma.
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The masculine plural loses its wāw
Emphatic Lām and the Heavy and Light Nūn Paradigms
In يَفْعَلُونَ and تَفْعَلُونَ, the wāw signals masculine plural and the nūn is the indicative inflectional ending. Before the emphatic nūn, both the inflectional nūn and the plural wāw disappear, but the ḍamma before the emphatic ending preserves the plural distinction.
يَفْعَلُونَ ← لَيَفْعَلُنَّ — the men will certainly do.
تَفْعَلُونَ ← لَتَفْعَلُنَّ — you men will certainly do.
Compare singular لَيَفْعَلَنَّ: the singular has fatḥa before the nūn, while the masculine plural has ḍamma.