In plain words
Know that by which all this is pervaded to be indestructible. No one can bring about the destruction of this imperishable.
Word for word
But know that to be indestructible by which all this is pervaded; no one can bring about the destruction of this imperishable [one].
The commentators
Bhagavadgītābhāṣyam
किं पुनस्तत् , यत् सदेव सर्वदा इति ; उच्यते —
अविनाशि न विनष्टुं शीलं यस्येति । तुशब्दः असतो विशेषणार्थः । तत् विद्धि विजानीहि । किम् ? येन सर्वम् इदं जगत् ततं व्याप्तं सदाख्येन ब्रह्मणा साकाशम् , आकाशेनेव घटादयः । विनाशम् अदर्शनम् अभावम् । अव्ययस्य न व्येति उपचयापचयौ न याति इति अव्ययं तस्य अव्ययस्य । नैतत् सदाख्यं ब्रह्म स्वेन रूपेण व्येति व्यभिचरति, निरवयवत्वात् , देहादिवत् । नाप्यात्मीयेन, आत्मीयाभावात् । यथा देवदत्तो धनहान्या व्येति, न तु एवं ब्रह्म व्येति । अतः अव्ययस्य अस्य ब्रह्मणः विनाशं न कश्चित् कर्तुमर्हति, न कश्चित् आत्मानं विनाशयितुं शक्नोति ईश्वरोऽपि । आत्मा हि ब्रह्म, स्वात्मनि च क्रियाविरोधात् ॥ १७ ॥
In plain words
But what is this thing that simply is, and is always? Here is the answer —
'Not-to-perish' — that is simply what avināśi means: something whose very nature it is never to be destroyed. The word 'tu' is there to draw a line between this and everything unreal. 'Know that' — know what? By which all this visible world is stretched through and pervaded, by Brahman, called Sat — and even space itself is folded into that pervading, the way space pervades every pot and jar. Destruction here just means ceasing to appear, ceasing to be at all. And 'imperishable' means exactly this: it never departs, never grows, never shrinks, never comes or goes — that is what makes it imperishable. This Brahman called Sat never departs from its own nature, because it has no parts to come apart, unlike a body. Nor does it lose anything of 'its own', because nothing stands apart from it to be lost. Devadatta can be diminished by the loss of his wealth — but Brahman is never diminished that way. So no one can bring about the destruction of this imperishable Brahman; no one — not even Īśvara — has the power to destroy the Self. Because the Self is Brahman itself, and an action can never turn back upon the very one performing it.
Close to the Sanskrit
But what is that, which is 'existent alone, always' — thus? It is said —
'Avināśi' [means]: not-to-perish is whose nature — thus [is meant]. The word 'tu' has the purpose of a qualifier distinguishing [the Self] from the unreal. 'Tat' [means] that; 'viddhi', know — understand. What? By which all this world is stretched, pervaded, by Brahman called Sat, together with [its] space — as pots and so on are pervaded by space. 'Vināśa' [means] non-appearance, non-existence. 'Of the imperishable': it does not depart — does not undergo increase and decrease — thus 'imperishable', of that imperishable one. This same Brahman called Sat does not depart in its own form, does not deviate, because it is without parts, unlike the body and so on. Nor [does it depart] through what is its own, because there is nothing that is 'its own'. As Devadatta departs [diminishes] through loss of wealth, but Brahman does not depart thus. Therefore no one is able to bring about the destruction of this imperishable Brahman; no one is able to destroy the Self, even the Lord. For the Self is Brahman, and because action upon one's own self is a contradiction.