Language is the most important means of communication between human beings, which, due to its nature and for various reasons, undergoes changes and transformations over time. Language transformations occur in its sounds, syntax, and vocabulary. Many common words in Persian today have undergone these changes throughout history. Some of these changes occur in Middle Persian, and some others occurred in Dari Persian, and continue to this day in some dialects. Among these changes is the deletion and addition of the consonant t in some contexts, which include words with different grammatical identities. This article deals with the reduction and increase of the phoneme t after s and š in some contexts and its causes. This phenomenon occurs due to the proximity of the phonemes of these consonants. In the first part of this article, we will discuss the increase of the non-derivative consonant /t/, and in the second part, we will discuss its deletion.

The author's goal in this article is to discover the laws governing these words by collecting words from ancient texts in which these processes are evident, and to draw the attention of researchers to these words and their phonological developments - which have received little attention so far. It is worth mentioning that some correctors, due to their unfamiliarity with these developments, have incorrectly placed the transformed words in the margin and inserted another word in the text.