Short vocals E, I, U i.e. (Fatha, Kassra and Damma) can be doubled, so this duplication of the vocals is called TENVIN. Tenvin appears only at the end of the word, i.e. with unspecified nouns. The doubled vocal is written by adding another symbol next to the existing, and it is pronounced by adding “N” in the pronunciation of the double vocal. According to that we have tenvin Fatha (E) – En, tenvin Kassre (I) – In, a tenvin Damma (U) – Un.
The letters i.e. harfs can be also doubled like the vocals. But they are not written twice, we put a small sign similar to the english letter “w” above the letter. The symbol is called SHADDA and when it is placed above a letter that letter should be doubled in the pronunciation or pronounced stronger or longer.