परस्तस्मात्तु भावोऽन्योऽव्यक्तोऽव्यक्तात्सनातनः।यः स सर्वेषु भूतेषु नश्यत्सु न विनश्यति॥
paras tasmāt tu bhāvo ’nyo ’vyakto ’vyaktāt sanātanaḥyaḥ sa sarveṣu bhūteṣu naśyatsu na vinaśyati
Word by word
The verse says
Beyond that unmanifest is another unmanifest, older, which does not perish when all beings perish.
In plain words
Two unmanifests, and the difference matters.
The first is the night — where things go when they stop appearing, and come back from.
The second is not a place things go. It is what all of that is happening inside, and it does not take part.
Where you've felt this · in study
Turn off the lights in a room you know well. Everything disappears, and nothing has moved. When the lights come back the chair is exactly where it was, which tells you something you had never needed to think about: the chair was not made of light, even though everything you knew about it arrived that way.
Going deeper
Distinguishing between what is not appearing and what is not there is the whole work of this verse.
The cycle of day and night takes care of appearance. Things show up, things withdraw, and both are events. What is named here is older than either, sanātana, and it is not the sort of thing that has events. When every being ends, it is not one of the things that ended.
This is the same distinction the book has been making since Chapter 2, where the self was said not to be killed when the body is killed, but it is being made at the scale of everything rather than one person.
What it gives you is a place to stand. Whatever else is true about your situation, you are not made entirely of the parts of it that are currently switching on and off.