In plain words
For death is certain for one who is born, and birth is certain for one who dies. Therefore you should not grieve over what cannot be avoided.
Word for word
For of the born, death is certain, and certain is birth of the dead; therefore, in an unavoidable matter, you ought not to grieve.
The commentators
Bhagavadgītābhāṣyam
तथा च सति —
जातस्य हि लब्धजन्मनः ध्रुवः अव्यभिचारी मृत्युः मरणं ध्रुवं जन्म मृतस्य च । तस्मादपरिहार्योऽयं जन्ममरणलक्षणोऽर्थः । तस्मिन्नपरिहार्येऽर्थे न त्वं शोचितुमर्हसि ॥ २७ ॥
In plain words
And since that is how it stands —
Whoever is born — whoever has taken up a birth — will die; that is fixed, and it never fails. And whoever has died will be born; that too is fixed. So this whole affair, marked at both ends by birth and death, is not something anyone can ward off. And in a matter no one can ward off, grieving does not befit you.
Close to the Sanskrit
And this being so —
For of one born — of one who has obtained a birth — death, that is, dying, is certain, unfailing; and birth is certain for one who has died. Therefore this matter, whose marks are birth and death, cannot be warded off. In that matter which cannot be warded off, you ought not to grieve.