Vowels: Nunation
The "N" sound
The three short vowels dealt with in the lesson about vowels have their doubled forms, i.e, ــً ــٌ ــٍ. The doubled vowels are pronounced like their single equivalents but with an "N" sound appended. This "N" sound is only pronounced and never rendered in writing. So, whereas the 3 single vowels are pronounced as a, oo and i, the doubled vowels are pronounced as an, oon and in (like in anti, wounded and pin).
So a ب with the doubled vowels will be written as بً بٌ بٍ but pronounced as بَنْ بُنْ بِنْ. In fact, بً will be written as بًا, i.e, with an additional ا; this will be dealt with below.
Where are they found?
The doubled vowels only come at the end of words; you will never come across a doubled vowel above/below other than over the last letter of a word.
With nouns/adjectives only
The doubled vowel is only placed on the last letter of a word, and this word should necessarily be a noun/adjective. No doubled vowels with verbs or prepositions or particles. The doubled vowel is in fact a sign that a word is a noun and not a verb or particle.
The additional ا
To the word ending in ــً is added an ا, so with the 3 doubled vowels above/below ش we will have شًا شٌ شٍ . However, if the ــً is on a ة, no ا is added and the word ends in ـةً.