For the ta of the niṣṭhā there is substituted na after a root ending in ā and commencing with a conjunct consonant, if the latter contains a semi-vowel.,
Thus from द्रा we have प्रद्राणः, प्रद्राणवान् ॥ ग्लानः, ग्लानवान् from ग्ला ॥ These roots द्रा and ग्ला end in आ, have a conjunct consonant in the beginning, one of which is a semi-vowel र and ल ॥
Why do we say \beginning with a double-consonant\? Observe यातः, यातवान् ॥
Why do we say \ending in आ\? Observe च्युतः, च्युतवान्; प्लुतः, प्लुतवान् ॥
Why do we say \after a root\? Observe निर्यातः, निर्वातः ॥ For the roots या and वा do not begin with a conjunct consonant, and that which is a conjunct i.e. र्या and र्वा is not a root, hence the rule does not apply.
Why do we say \having a यण् or semi-vowel\? Observe स्नातः, स्नातवान् ॥,