V. S. Apte, The Practical Sanskrit–English Dictionary. Look up any word of the Gita in Devanagari, IAST or Harvard-Kyoto, or browse the headwords letter by letter.
tatraivaṁ sati kartāramātmānaṁ kevalaṁ tu yaḥ, — That being so, he verily who, owing to ignorant understanding, looketh on the pure Self as the doer, he of perverted intelligence, seeth not. 18·16
सतिsatilocative singular of the neuter form of the present participial adjective sat (being so).