In plain words
Knowing this, seekers of liberation in ancient times also performed action. Therefore perform action alone, as the ancients did before you.
Word for word
Knowing thus, action was done also by seekers of liberation of old; therefore you, do action alone, done by the ancients earlier.
The commentators
Bhagavadgītābhāṣyam
‘नाहं कर्ता न मे कर्मफले स्पृहा’ इति —
एवं ज्ञात्वा कृतं कर्म पूर्वैः अपि अतिक्रान्तैः मुमुक्षुभिः । कुरु तेन कर्मैव त्वम् , न तूष्णीमासनं नापि संन्यासः कर्तव्यः, तस्मात् त्वं पूर्वैरपि अनुष्ठितत्वात् , यदि अनात्मज्ञः त्वं तदा आत्मशुद्ध्यर्थम् , तत्त्वविच्चेत् लोकसङ्ग्रहार्थं पूर्वैः जनकादिभिः पूर्वतरं कृतं न अधुनातनं कृतं निर्वर्तितम् ॥ १५ ॥
In plain words
‘I am not the doer, and I have no craving for what action bears’ — thus —
This is what earlier seekers of liberation understood too, before they passed on, and it is why they acted. So: act — act alone, and nothing else, not silent sitting, not even renunciation. Act, because those before you acted too — if you do not yet know the Self, act for the sake of purifying yourself; if you already know the truth, act for the welfare of the world. This is exactly what Janaka and others before you did, long ago — this is nothing newly invented.
Close to the Sanskrit
‘Not I the doer; not of me, in the fruit of action, is craving’ — thus —
Thus, having known this, action was performed even by former, departed seekers of liberation. 'Accordingly, act — action alone, you [should do]' — neither silent sitting nor even renunciation is to be adopted; therefore you [should act], since it was practiced even by those before you: if you are not a knower of the Self, then for the sake of self-purification; if you are a knower of the truth, then for the sake of the welfare of the world — as it was done, earlier, by those before you, Janaka and others; not something recent, newly accomplished. || 15 ||