a. In the vast majority of cases, the sibilant is the final of the tense-stem, and the inflection is like that of an imperfect of the first or non-a-conjugation.

b. And these, again, fall into two nearly equal and strongly marked classes, according as the sibilant is added immediately to the final of the root, or with an auxiliary vowel इ i, making the tense-sign इष् iṣ. Finally, before this इष् iṣ the root is in a very small number of cases increased by a स् s, making the whole addition सिष् siṣ.