यदा वै करोत्यथ निस्तिष्ठति नाकृत्वा निस्तिष्ठति कृत्वैव निस्तिष्ठति कृतिस्त्वेव विजिज्ञासितव्येति कृतिं भगवो विजिज्ञास इति ॥ ७.२१.१ ॥
In plain words
"When a man acts, then he is steadfast; one who does not act is not steadfast. Only one who acts is steadfast. But one must seek to understand action." "Sir, I seek to understand action."
The commentators
Chāndogyopaniṣadbhāṣyam· bhāṣya
यदा वै करोति । कृतिः इन्द्रियसंयमः चित्तैकाग्रताकरणं च । सत्यां हि तस्यां निष्ठादीनि यथोक्तानि भवन्ति विज्ञा- नावसानानि ॥
In plain words
When one truly practices—and practice means mastering the senses and focusing the mind—then, and only then, do the stages of commitment unfold as described, culminating in direct realization.
Close to the Sanskrit
When indeed one performs action; action is the control of the senses and the making of one-pointedness of the mind. Because when that is present, the stages beginning with commitment, as described, manifest, culminating in direct realization.