What is the secret behind the use of "khalideen" (abiding eternally) in verse 13 of Surat An-Nisa and "khalidan" (abiding eternally) in verse 14 of the same Surah?

Perhaps the wisdom behind this is what Abu Al-Saud pointed out in his Tafsir (exegesis): that the plural form (khālidīn - abiding) came with regard to the collectivity of (man - whoever) according to its meaning, while the singular pronoun (khālidan - abiding) came with regard to its singular form in wording.