In plain words
But beyond that unmanifest is another unmanifest bhāva, eternal, which does not perish when all beings perish.
Word for word
But other than that, beyond the unmanifest, [is] another unmanifest, eternal being, which, when all beings perish, does not perish.
परस्तस्मात् तु भावोऽन्योऽव्यक्तोऽव्यक्तात् सनातनःयः स सर्वेषु भूतेषु नश्यत्सु न विनश्यति
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
यत् उपन्यस्तम् अक्षरम् , तस्य प्राप्त्युपायो निर्दिष्टः ‘ओमित्येकाक्षरं ब्रह्म’(भ. गी. ८ । १३) इत्यादिना । अथ इदानीम् अक्षरस्यैव स्वरूपनिर्दिदिक्षया इदम् उच्यते, अनेन योगमार्गेण इदं गन्तव्यमिति —
परः व्यतिरिक्तः भिन्नः ; कुतः ? तस्मात् पूर्वोक्तात् । तु—शब्दः अक्षरस्य विवक्षितस्य अव्यक्तात् वैलक्षण्यविशेषणार्थः । भावः अक्षराख्यं परं ब्रह्म । व्यतिरिक्तत्वे सत्यपि सालक्षण्यप्रसङ्गोऽस्तीति तद्विनिवृत्त्यर्थम् आह — अन्यः इति । अन्यः विलक्षणः । स च अव्यक्तः अनिन्द्रियगोचरः । ‘परस्तस्मात्’ इत्युक्तम् ; कस्मात् पुनः परः ? पूर्वोक्तात् भूतग्रामबीजभूतात् अविद्यालक्षणात् अव्यक्तात् । अन्यः विलक्षणः भावः इत्यभिप्रायः । सनातनः चिरन्तनः यः सः भावः सर्वेषु भूतेषु ब्रह्मादिषु नश्यत्सु न विनश्यति ॥ २० ॥
In plain words
We have already been told how to reach the Imperishable — through 'Om, the one syllable, is Brahman' (Bhagavad Gītā 8.13) and the verses around it. Now, wanting to show what that Imperishable actually is in itself, this is said: that it is to be reached by this path of yoga —
'Beyond' means different, set apart from what came before. From what, exactly? From that unmanifest state already discussed. The word 'but' is there precisely to mark that this Imperishable is not the same kind of thing as that earlier unmanifest. 'This entity' means the supreme Brahman called the Imperishable. Even after being told it is distinct, someone might still assume it shares the same nature as before — so, to rule that out, the verse adds 'other.' 'Other' here means genuinely different in kind, not just numerically separate. And it too is unmanifest, beyond anything the senses could ever reach. We are told it is 'beyond that' — beyond what? Beyond that same unmanifest already discussed, the one that is nothing but ignorance itself, the very seed from which every being sprouts. It is truly other, truly different — that is the point. Eternal, ageless — this reality does not perish, even when every being, Brahmā included, perishes.
Close to the Sanskrit
The imperishable that was set forth — its means of attainment has been indicated by 'Om, the one syllable, is Brahman' (Bhagavad Gītā 8.13) and so on. Now, at this point, with the desire to indicate the very nature of the imperishable itself, this is said: 'this is to be reached by this path of yoga' —
'Beyond': different, distinct; from what? From that previously mentioned. The word 'tu' serves to mark a distinction from the unmanifest state of the Imperishable now under discussion. 'Bhāva' means that supreme Brahman called the Imperishable. Even though it has been said to be distinct, there is still the risk that it might be assumed to share the same characteristics; to remove that misunderstanding he says 'other' — thus. 'Other' means dissimilar in kind. And it too is unmanifest, beyond the range of the senses. It has been said 'beyond that'; beyond what, again? Beyond that previously mentioned unmanifest, characterized by ignorance, which is the seed of the aggregate of beings. It is 'other,' a dissimilar entity — this is the sense. Eternal, primeval — that entity, when all beings, beginning with Brahmā, perish, does not perish.