The word 'atha' is used here in the sense of conceding the point. Even if you think this Self to be perpetually born, taking it, by the common opinion, to be born again and again with the arising of each body, and likewise think it perpetually dying, dead with the destruction of each body: even then, even granting it to be so, O mighty-armed one, you should not grieve in this way, since for whatever is born destruction is certain, and for whatever is destroyed birth is certain. And since that is so.

This is a brief sub-gloss. For a fuller reading of this verse, see Madhusūdana, Śaṅkara, or Rāmānuja above.

But if you think this self to be the very body, ever born and ever dying, and not something distinct from the body of the character described, even then you ought not to grieve so excessively, for the origination and destruction of the body, whose nature is transformation, cannot be avoided.

This is a brief sub-gloss. For a fuller reading of this verse, see Madhusūdana, Śaṅkara, or Rāmānuja above.

Granted that the self is eternal in this way; even so, birth and death, which consist in union with and separation from a body, do still occur. To that Krishna replies with 'and even if you think it'.