إِنَّ and its 'sisters' are particles that have a special effect on the nominal sentence when they precede it.
When any of these particles precedes the nominal sentence, the subject (الْمُبْتَدَأُ) of that sentence is then called اسْمُ إِنَّ and is put in the accusative case (مَنْصُوبٌ), and the predicate of that subject خَبَرُ إِنَّ remains in the nominative case.
Ex. الْمَطَرُ غَزِيرٌ/ إنَّ المَطَـرَ غَزِيـرٌ
The sisters of إِنَّ are:
- إِنَّ ـ أَنَّ intensifiers. Example: إنَّ مالِكًا عالِمٌ - كَأَنَّcomparison/as if Ex. كأنَّ العلمَ نورٌ - لَكِنَّbut Ex. النّحْوُ سَهْلٌ لكِنَّ قَواعِدَهُ مُتَشَعِّبَةٌ - لَيْتَ to express a wish/ if only. Ex. ليْتَ الحَبِيبَ قادِمٌ - ظَنٌّ: لَعَلَhypothesis (maybe). Ex. لعلَّ المَطَرَ نازِلٌ
