Ordinary dual imperfect forms have an alif followed by an indicative inflectional nūn. With heavy emphasis, that inflectional nūn disappears and a heavy nūn with kasrah follows the retained alif.
يَفْعَلَانِ ← لَيَفْعَلَانِّ — the two males will certainly do.
تَفْعَلَانِ ← لَتَفْعَلَانِّ — the two females will certainly do, or you two will certainly do.
Retaining the alif preserves dual number. The textbook's light-nūn column has dashes for these dual positions, so do not invent a parallel light form.
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Dual forms retain the alif
Emphatic Lām and the Heavy and Light Nūn Paradigms
Ordinary dual imperfect forms have an alif followed by an indicative inflectional nūn. With heavy emphasis, that inflectional nūn disappears and a heavy nūn with kasrah follows the retained alif.
يَفْعَلَانِ ← لَيَفْعَلَانِّ — the two males will certainly do.
تَفْعَلَانِ ← لَتَفْعَلَانِّ — the two females will certainly do, or you two will certainly do.
Retaining the alif preserves dual number. The textbook's light-nūn column has dashes for these dual positions, so do not invent a parallel light form.