Writing in Arabic: sukuun, shadda, taa marbuuTa
The sukuun, meaning absence, is merely a grammatical marker indicating the absence of a short vowel following a consonant. All consonants carry a grammatical marker - such as a short vowel - even if that marker, like sukuun, indicates an absence.
The shadda is another grammatical maker, indicating the double length of a consonant. A shadda may never occur at the beginning of a word - it may only sit on a letter in the medial or final position. Furthermore, a shadda may never be directly followed by a consonant, as three consonants in a row, uninterrupted by a short or long vowel, may never occur in Arabic.
taa marbuuTa ة is not considered a letter of the alphabet since its function is primarily grammatical. This symbol indicates the feminine gender, and is related to the letter ت . The fatHa short vowel that precedes ة is always pronounced. When you hear a noun that ends with a “fatHa sound” you can usually assume that it is feminine, spelled with a ة at the end.