In plain words
The word 'Sat' is used when what was not comes to be and when one who was not good becomes good, and likewise, Partha, the word 'Sat' is used of an auspicious action.
Word for word
In coming to be and in becoming good, this [word] 'Sat' is used; likewise in a praiseworthy act, O Pārtha, the word 'Sat' is employed.
सद्भावे साधुभावे च सदित्येतत् प्रयुज्यतेप्रशस्ते कर्मणि तथा सच्छब्दः पार्थ युज्यते
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
ओन्तच्छब्दयोः विनियोगः उक्तः । अथ इदानीं सच्छब्दस्य विनियोगः कथ्यते —
सद्भावे, असतः सद्भावे यथा अविद्यमानस्य पुत्रस्य जन्मनि, तथा साधुभावे च असद्वृत्तस्य असाधोः सद्वृत्तता साधुभावः तस्मिन् साधुभावे च सत् इत्येतत् अभिधानं ब्रह्मणः प्रयुज्यते अभिधीयते । प्रशस्ते कर्मणि विवाहादौ च तथा सच्छब्दः पार्थ, युज्यते प्रयुज्यते इत्येतत् ॥ २६ ॥
In plain words
The use of the words oṃ and tat has been explained. Now the use of the word sat is taken up —
'In the sense of existence' -- meaning, the coming-into-being of what did not exist before -- like when a son is born who was not there before: his birth is what 'sat' marks. And likewise, 'in the sense of goodness': when someone who used to behave badly, who was not virtuous, turns virtuous -- that turning itself is called 'sadhubhava,' and it is in that very turning toward goodness that this word 'sat,' this name of Brahman, gets used. And in any praiseworthy act too, like a wedding, the same word 'sat' is used the same way, Arjuna -- that is what is meant.
Close to the Sanskrit
Of the two words oṃ and tat, the application has been stated. Now, at this point, of the word sat the application is being told —
'In the sense of existence' -- 'of the non-existent, in the coming-into-being,' as in the birth of a son who did not previously exist; likewise, 'in the state of goodness' too -- the state of good conduct of one who was of bad conduct, not virtuous, i.e. virtuousness, [called] 'sadhubhava' -- in that state of becoming virtuous too, this designation 'sat' of Brahman is employed, is used. In a praiseworthy action, such as marriage etc. too, likewise the word 'sat,' O Partha, is applied, is employed -- this [is the meaning].