474. yadā.

yadā is a temporal conjunction = our „when.” Its demonstrative which is generally not omitted, is tadā „then.” Panc. 303 yadā rāmo rājāsīt tadāham… anena pathā samāyātaḥ, Mṛcch. I, p. 55 yadā tu bhāgyakṣayapīḍitāṃ daśāṃ naraḥ kṛtāntopahitāṃ prapadyate / tadāsya mitrāṇy api yānty amitratāṃ cirānurakto ‘pi virajyate janaḥ.

yadā repeated is of course = »whenever.” Kathâs. 25, 216 tasmān niśi ca bhūyo ‘pi tvam eṣyasi yadā yadā / tadā tadā vaṭataror mūlāt prāpsyasi māmitaḥ »at the very time that.” Ven. I, p. 24 kuruṣu tāvad asaṃdheyatā tadaiva niveditā yadaivāsmābhir ito vanaṃ gacchaddhiḥ sarvair eva kurukulasya nidhanaṃ pratijñātam. — yadāprabhṛti »since.” R. 2, 116, 13 tvaṃ yadāprabhṛti hy asminn āśrame tāta vartase / tadāprabhṛti rakṣāṃsi viprakurvanti tāpasān (for the hermits are being vexed by the râxasas since the time, that you stay here).

474. yadā

yadāは時間を表す接続詞（英語でいうwhenに相当）である。その指示代名詞であるtadā（そのとき、then）はふつう省略されない。
Panc. 303 yadā rāmo rājāsīttadāham… anena pathā samāyātaḥ
Mṛcch. I, p. 55 yadā tu bhāgyakṣayapīḍitāṃ daśāṃ naraḥ kṛtāntopahitāṃ prapadyate / tadāsya mitrāṇyapi yānty amitratāṃ cirānurakto ‘pi virajyate janaḥ

繰り返されるyadāはもちろん「いつでも」と等しい。
Kathâs. 25, 216 tasmān niśi ca bhūyo ‘pi tvameṣyasi yadā yadā / tadā tadā vaṭatarormūlātprāpsyasi māmitaḥ »at the very time that.”
Ven. I, p. 24 kuruṣu tāvad asaṃdheyatā tadaiva niveditā yadaivāsmābhirito vanaṃ gacchaddhiḥ sarvair eva kurukulasya nidhanaṃ pratijñātam

yadāprabhṛtiは"since"：
R. 2, 116, 13 tvaṃ yadāprabhṛti hy asminnāśrame tāta vartase / tadāprabhṛti rakṣāṃsi viprakurvanti tāpasān (for the hermits are being vexed by the râxasas since the time, that you stay here)