The affix yat comes after roots gad 'to eat', mad 'to be mad', car 'to go, to eat' and yam 'to restrain', when these roots do not take any preposition.,
Thus गद्यम् 'what is to be spoken, i.e prose'; मद्यम् 'wine'; चर्यम् 'what is to be followed'; यम्यम् 'what is to be restrained'.
Why do we say 'when not preceded by a preposition'? When compounded with an upasarga, these roots will take ण्यत्; thus प्रगाद्यम्; प्रमाद्यम् &c.
The root यम् would have taken the affix यत् by sutra 98 also, as it ends in म and is preceded by अ; its separate enumeration in this sutra is in order to show, that the compound verb यम् will not take the affix यत्. Therefore with regard to यम्, this is a niyama or restrictive rule and not a vidhi or original rule.
Vart:- The root चर when preceded by the preposition आङ् takes the यत्, affix when the sense is not that of a preceptor; thus आचर्यो देशः 'an approachable or communicable country'. But when the sense is that of a teacher the root takes ण्यत्; thus आचार्यः 'a preceptor',