In plain words
Even if you think the self is repeatedly born and repeatedly dies, even then, mighty-armed one, you should not grieve in this way.
Word for word
And if you think this [one] constantly born or constantly dead, even so, O mighty-armed, you ought not to grieve thus.
The commentators
Bhagavadgītābhāṣyam
आत्मनः अनित्यत्वमभ्युपगम्य इदमुच्यते —
अथ च इति अभ्युपगमार्थः । एनं प्रकृतमात्मानं नित्यजातं लोकप्रसिद्ध्या प्रत्यनेकशरीरोत्पत्ति जातो जात इति मन्यसे तथा प्रतितत्तद्विनाशं नित्यं वा मन्यसे मृतं मृतो मृत इति ; तथापि तथाभावेऽपि आत्मनि त्वं महाबाहो, न एवं शोचितुमर्हसि, जन्मवतो जन्म नाशवतो नाशश्चेत्येताववश्यम्भाविनाविति ॥ २६ ॥
In plain words
Granting that the Self is impermanent, this is what is said —
'Even so' means: let's grant your view for the sake of argument. Say you really do think the Self keeps getting born -- one body after another, and every single time you call it 'birth, birth again' -- or that it keeps dying, and every time you call it 'death, death again.' Fine. Even on those terms, mighty-armed one, you still have no reason to grieve like this -- because if something is born, its death was always coming, and if something dies, its birth was always the reason why. Neither one is avoidable, so neither is worth this grief.
Close to the Sanskrit
Having conceded the impermanence of the Self, this is said —
'Atha ca' means 'granting [the opposite], for argument's sake.' [Even if] you think that this aforementioned Self is constantly born -- by common repute, with respect to each of its many bodily births, thinking 'born, born' [each time] -- or likewise think it constantly dies with each and every destruction, thinking 'dead, dead' [each time]; even so, even in that case, regarding the Self, O mighty-armed one, you should not grieve like this, for birth, for one who has taken birth, and death, for one subject to death, are both inevitable.