In plain words
Sattva attaches one to happiness, Bharata; rajas to action; but tamas, veiling jñāna, attaches one to heedlessness as well.
Word for word
Sattva attaches to happiness, rajas to action, O Bhārata; but tamas, having veiled jñāna, attaches to negligence too.
सत्त्वं सुखे सञ्जयति रजः कर्मणि भारतज्ञानमावृत्य तु तमः प्रमादे सञ्जयत्युत
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
पुनः गुणानां व्यापारः सङ्क्षेपतः उच्यते —
सत्त्वं सुखे सञ्जयति संश्लेषयति, रजः कर्मणि हे भारत सञ्जयति इति अनुवर्तते । ज्ञानं सत्त्वकृतं विवेकम् आवृत्य आच्छाद्य तु तमः स्वेन आवरणात्मना प्रमादे सञ्जयति उत प्रमादः नाम प्राप्तकर्तव्याकरणम् ॥ ९ ॥
In plain words
Once more, and briefly this time, what the guṇas actually do —
Sattva attaches a person to pleasure; rajas, to action — the same verb applies to both, even though it's only spelled out once. But tamas works differently: it first covers over the very discernment that sattva produces, blotting it out through its own natural fog, and then it attaches a person to sheer carelessness instead. And carelessness, precisely, means this: a duty comes right up to you, ready to be done, and you simply don't do it.
Close to the Sanskrit
Again, the activity of the guṇas is stated in brief —
Sattva causes attachment, causes fusion, to pleasure; rajas, to action, O Bhārata — the verb 'causes attachment' carries over here as well. But tamas, having covered, having obscured, the discrimination produced by sattva, by its own veiling nature, causes attachment to negligence; and negligence means the non-performance of a duty that has already presented itself.