The weak ending appears differently across the taught forms.
FormCategory
اِسْلَنْقَىٰActive past
يَسْلَنْقِيْActive Mudhaari'
مُسْلَنْقٍIsm Fa'il
اِسْلَنْقِImperative
لَا تَسْلَنْقِProhibitive
Learn each form as it is taught rather than forcing the same ending into every slot.