In plain words
These two paths, the bright and the dark, are regarded as the eternal courses of the world. By one there is no return; by the other one returns again.
Word for word
These bright and dark paths of the world are indeed held eternal; by one [a person] goes to non-return, by the other returns again.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
शुक्लकृष्णे शुक्ला च कृष्णा च शुक्लकृष्णे, ज्ञानप्रकाशकत्वात् शुक्ला, तदभावात् कृष्णा ; एते शुक्लकृष्णे हि गती जगतः इति अधिकृतानां ज्ञानकर्मणोः, न जगतः सर्वस्यैव एते गती सम्भवतः ; शाश्वते नित्ये, संसारस्य नित्यत्वात् , मते अभिप्रेते । तत्र एकया शुक्लया याति अनावृत्तिम् , अन्यया इतरया आवर्तते पुनः भूयः ॥ २६ ॥
In plain words
'Bright and dark' — these two words together make 'the bright-and-dark pair': bright, because it is lit up by knowledge; dark, because that light is simply absent. These two, bright and dark, are exactly what the verse means by 'the two paths' — but paths belonging only to those actually qualified for the pursuit of knowledge or ritual action, not to absolutely everyone alive. 'Eternal' means precisely that: since this whole cycle of transmigration is itself beginningless and endless, these two paths are held, regarded, as eternal too. And of the two: by the bright one, a person goes to no return at all; by the other, the different one, a person comes back again, over and over.
Close to the Sanskrit
'Śuklakṛṣṇe': bright and dark together are 'bright-and-dark' — bright, because of illuminating knowledge; dark, because of the absence of that. These two, bright and dark, indeed, are the 'two paths' — of the world, that is, of those qualified for knowledge and action; these two paths do not pertain to absolutely everyone in the world. 'Śāśvate' means eternal, since transmigratory existence is eternal; 'mate' means held to be, regarded. There, by one, the bright one, a person goes to non-return; by the other, the different one, one returns, again, repeatedly.
Gītā Vāhini
“ఉత్తరాయణ మార్గము పవిత్రమైన జ్ఞాన ప్రకాశమగుటచే దానిని ‘శుక్ల మార్గ’మనియును, దక్షిణాయన మార్గము తమోమయమై, జ్ఞానప్రకాశము కాక అంధకారమయమైనదగుట చేత దానిని ‘కృష్ణ మార్గ’మనియును చెప్పబడినది. మొదటి మార్గమైన శుక్లమార్గమున పయనము చేసిన వారు (అనగా, శరీరమును వదలి వెళ్ళినవారు), జనన మరణము లేక (అనగా, తిరిగి దేహములు ధరించక) బ్రహ్మానంద నిలయమైన, మోహరహితమైన స్థానమగు ‘మోక్షము’ను పొందగలరనియు, రెండవ మార్గమైన కృష్ణ మార్గమున ప్రయాణమైన వారికి (దేహమును వీడిన వారికి), తిరిగి జనన మరణ కారణమగు దేహము ప్రాప్తించుననియు తెలిసికొనవలెను.”
In plain words
“The path of Uttarāyaṇa is the holy light of Jñāna, and so it is called the ‘Śukla mārga’. The path of Dakṣiṇāyana is full of tamas; it is not the light of Jñāna but is full of darkness, and so it is called the ‘Kṛṣṇa mārga’. Understand this: those who journey on the first path, the Śukla mārga (that is, those who leave the body and depart), are free of birth and death (that is, they do not take on bodies again) and can attain ‘Mokṣa’, the state that is the abode of Brahmānanda, free of moha; and to those who set out on the second path, the Kṛṣṇa mārga (those who leave the body), a body, the cause of birth and death, comes again.”