उपसर्गाः क्रियायोगे

The words प्र etc. get the designation of उपसर्ग or prepositions, when in composition with a verb.
As प्र + नयति = प्रणयति. Here the dental न is changed into cerebral ण, because of the प्र getting the designation of upasarga (See sutra 8.4.14). Similarly परिणयति, प्रणायकः, परिणायकः.

Why do we say when in composition with a verb? When in composition with a noun they are not called upasargas. As प्रगतो नायकोऽस्माद् देशात् = प्रनायकोदेशः a country destitute of a leader. Here न is not changed into ण. Thus while प्रणायकः means 'a leader' the term प्रनायकः means destitute of a leader; though both have the same radical elements :-- pra in one is an upasarga, in the other a Nipata pure and simple.

Vart:- The word मरुत् should be included in the list of upasargas. As मरुद्भिर्दत्तः = मरुत्तः given by Marut. मरुत् + दा + क्त = मरुत् + त् + त 7.4.47. Here Marut being treated as an upasarga, though it does not end with a vowel, the दा is replaced by त by rule 7.4.47 which declares 'त the substitute of दा which is called ghu when it is preceded by an upasarga that ends in a vowel, and is followed by an affix beginning with त which has an indicatory k.'

It might be objected, that as मरुत् does not end with a vowel, sutra 7.4.47 does not apply. To this we say that otherwise the giving the designation of upasarga to मरुत् is superfluous; and in order that this should not be so, the fact of its not ending with a vowel is overlooked.

Vart:- The particle श्रत् should be included in the list of upasargas. Thus श्रत् + धा + अङ् = श्रद्धा 3.3.106. Here because श्रत् is treated as an upasarga, that sutra 3.3.106 is made applicable.