الْيَقِيْنِ

A word in the Arabic language will either be 1) a verb or 2) a noun or 3) a particle – الْيَقِيْنِ is a noun

A noun may either be 1) Definite or indefinite 2) Masculine or feminine 3) Singular, dual or plural 4) Nominative, accusative or genitive

الْيَقِيْنُ is the natural state of this definite noun and it changes to الْيَقِيْنِ due to: 1) a preposition that precedes it or 2) a possessed noun that precedes it or 3) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

Sure, certainty, death, inevitable

To be certain, obvious

مَعْرِفَةٌمُفْرَدٌمُذَكَّرٌمَجْرُوْرٌي ق نبَاب سَمِعَفَعِيْلٌيَقِنَ يَيْقَنُاسْمٌاِسْمُ الْمُبَالَغَةِ
DefiniteSingularMasculineGenitiveRootCertaintyPatternسNounNoun of intensity 
اسْمٌ فَاعِلٌ
Active participle
Surah/Below are the 3 verses in the holy Quran containing the definite, possessive nounالْيَقِيْنِ
Verse
69-51
69-51
69-51
102-5Nay! If you only knew with a knowledge of certaintyكَلَّا لَوْ تَعْلَمُوْنَ عِلْمَ الْيَقِيْنِ
102-7Then you will certainly see it with the sight of certaintyثُمَّ لَتَرَوُنَّهَا عَيْنَ الْيَقِيْنِ