(س) يَقِنَ يَيْقَنُ يَقَنًاyaqina yayqanu yaqanan: to know firmly and beyond doubt. (this verb is not used in the Quran).
يَقِيْنٌyaqīn (on the pattern فَعِيْلٌfaʿīl): knowledge that admits no doubt; and metonymically death, since it is the most certain thing of all. ﴿وَاعْبُدْ رَبَّكَ حَتّٰى يَاْتِيَكَ الْيَقِيْنُ﴾ (Al-Ḥijr 15:99); ﴿اِنَّ هٰذَا لَهُوَ حَقُّ الْيَقِيْنِ﴾ (Al-Wāqiʿah 56:95)
Form IV (ifʿāl) اَيْقَنَ يُوْقِنُ اِيْقَانًاayqana yūqinu īqānan: to be certain; مُوْقِنٌmūqin pl. مُوْقِنُوْنَmūqinūna (active participle, ism fāʿil): one who holds certainty. ﴿وَبِالْاٰخِرَةِ هُمْ يُوْقِنُوْنَ﴾ (Al-Baqarah 2:4); ﴿وَفِي الْاَرْضِ اٰيٰتٌ لِّلْمُوْقِنِيْنَ﴾ (Adh-Dhāriyāt 51:20)
Form X (istifʿāl) اِسْتَيْقَنَ يَسْتَيْقِنُ اِسْتِيْقَانًاistayqana yastayqinu istīqānan: to become fully certain; مُسْتَيْقِنٌmustayqin pl. مُسْتَيْقِنُوْنَmustayqinūna (active participle, ism fāʿil). ﴿وَجَحَدُوْا بِهَا وَاسْتَيْقَنَتْهَآ اَنْفُسُهُمْ﴾ (An-Naml 27:14) — that is, their hearts were certain while their tongues denied; ﴿لِيَسْتَيْقِنَ الَّذِيْنَ اُوْتُوا الْكِتٰبَ﴾ (Al-Muddaththir 74:31); ﴿وَّمَا نَحْنُ بِمُسْتَيْقِنِيْنَ﴾ (Al-Jāthiyah 45:32)