TANWEEN (NUNATION) DOUBLE SHORT VOWELS SIGNS

If we put double Fathah  (Zabar)   ;    Kasra (Zer)     ;     Damma (Pesh)     the additional signs represents  letter "Nun" with Sakun (jazm)    which produces "N" sound.   thus "Nun" is not written but it is pronounced.
The following words may be written in two ways and rendered the same pronunciation..

                                 
             FuN                               FiN                            FaN

Practice Examples

ADVANCE READING

Tanwin (Nunation) which is an unvowelled and unwritten Nun coming at the end of a noun, and is indicated by the duplication of the final vowel symbol in the word 

Nouns and adjective when indefinite the double vowel signs are written