dha is substituted for ta or tha coming after jhaṣ (jha, bha, gha, ḍha, dha) but not after the root dhā (dadha).,
Thus from लभ् we have लब्धा, लब्धुम्, लब्धव्यम्, अलब्ध, अलब्धाः ॥
The भ् of लभ् is changed to ब by 8.4.53. From द्य्ह् — दोग्धा, दोग्धुम्, दोग्धव्यम्, अदुग्ध, अदुग्धाः ॥ The ह is changed to घ by 8.2.32, and then it is changed to ग by 8.4.53.
From लिह :- लेढा, लेढुम्, लेढव्यम्, अलीढ, अलीढाः ॥ In लेढ्धृ &c, the ह्, is changed to ढ by 8.2.31, and ध changed to ढ by 8.4.41, before which the preceding ढ is elided by 8.3.13.
From बुध् :- बोद्धा, बोद्धुम्, बोद्धव्यम्, अबुद्ध, अबुद्धाः ॥ For the ध of बुध् there is substituted द by 8.4.53.
Why do we say \but not after the root धा\? Observe धत्तः धत्थः ॥,