In plain words
Better one's own dharma, though imperfectly performed, than another's dharma well performed. Better to die in one's own dharma; another's dharma is fraught with danger.
Word for word
Better is one's own dharma, though lacking in merit, than another's dharma well performed; death in one's own dharma is better; another's dharma brings danger.
The commentators
Bhagavadgītābhāṣyam
तत्र रागद्वेषप्रयुक्तो मन्यते शास्त्रार्थमप्यन्यथा ‘परधर्मोऽपि धर्मत्वात् अनुष्ठेय एव’ इति, तदसत् —
श्रेयान् प्रशस्यतरः स्वो धर्मः स्वधर्मः विगुणः अपि विगतगुणोऽपि अनुष्ठीयमानः परधर्मात् स्वनुष्ठितात् साद्गुण्येन सम्पादितादपि । स्वधर्मे स्थितस्य निधनं मरणमपि श्रेयः परधर्मे स्थितस्य जीवितात् । कस्मात् ? परधर्मः भयावहः नरकादिलक्षणं भयमावहति यतः ॥
यद्यपि अनर्थमूलम् ‘ध्यायतो विषयान्पुंसः’(भ. गी. २ । ६२) इति ‘रागद्वेषौ ह्यस्य परिपन्थिनौ’(भ. गी. ३ । ३४) इति च उक्तम् , विक्षिप्तम् अनवधारितं च तदुक्तम् । तत् सङ्क्षिप्तं निश्चितं च इदमेवेति ज्ञातुमिच्छन् अर्जुनः उवाच ‘ज्ञाते हि तस्मिन् तदुच्छेदाय यत्नं कुर्याम्’ इति ॥ ३५ ॥
In plain words
And here a person driven by attraction and aversion reads even the scripture crooked, concluding: “Another's dharma is dharma too, so it should certainly be practised as well.” That is false —
Your own duty, even done poorly, even stripped of any polish, is better than someone else's duty, even if you did it beautifully. It really is better. In fact, dying while staying true to your own duty is better than living a long life inside someone else's. Why? Because taking up a duty that was never yours brings real danger with it — the kind of danger that leads somewhere as serious as hell. That's the reason.
The root of all this ruin has already been named. It was there in "a man who keeps dwelling on objects" (Gītā 2.62), and there again in "attraction and aversion are the two robbers waiting on his road" (Gītā 3.34). But it was said in scattered places, and never pinned down. Arjuna wants it in one line, settled: *this* is the thing, this and nothing else. So he asks — because once he knows what it is, he can set to work tearing it out.
Close to the Sanskrit
There, one impelled by attraction and aversion construes even the meaning of the scripture otherwise — thus: “Another's dharma also, because it is dharma, is indeed to be performed” — that is untrue —
Better, more praiseworthy, is one's own duty — one's own duty, even if deficient in quality, even if its good qualities have departed, as it is actually being performed — than another's duty, than another's duty well performed, accomplished with excellence, even so. Death of one established in one's own duty, dying, is better than the life of one established in another's duty. For what reason? Because another's duty brings danger, characterized by hell and the like — it brings danger; that is why.
Although the root of ruin has indeed been stated — by 'of a man dwelling on objects' (Gītā 2.62) and by 'for attraction and aversion are his waylayers' (Gītā 3.34) — that statement was scattered and left unsettled. Wishing to know that same thing concisely and definitely, as 'this alone is it,' Arjuna spoke, thinking: 'For once that is known, I could make the effort to cut it out.'