What principally distinguishes لَنْ يَفْعَلَ and لَيَفْعَلَنَّ?

Emphatic Lām and the Heavy and Light Nūn Paradigms

• The first negates the future; the second emphatically affirms it
• Both are past passives
• The first is plural; the second dual
• The first emphasises; the second negates