yad yad ācarati śreṣṭhas
tat tad evetaro janaḥ
sa yat pramāṇaḿ kurute
lokas tad anuvartate

Translation of Bhagavad Gita 3.21

Whatever action a great man performs, common men follow. And whatever standards he sets by exemplary acts, all the world pursues.

Commentary by Sri A.C. Bhaktivedanta Swami Prabhupada of Gaudiya Sampradaya:

People in general always require a leader who can teach the public by practical behavior. A leader cannot teach the public to stop smoking if he himself smokes. Lord Caitanya said that a teacher should behave properly before he begins teaching. One who teaches in that way is called acarya, or the ideal teacher. Therefore, a teacher must follow the principles of shastra (scripture) to teach the common man. The teacher cannot manufacture rules against the principles of revealed scriptures. The revealed scriptures, like Manu-samhita and similar others, are considered the standard books to be followed by human society. Thus the leader’s teaching should be based on the principles of such standard shastras. One who desires to improve himself must follow the standard rules as they are practiced by the great teachers. The Srimad-Bhagavatam also affirms that one should follow in the footsteps of great devotees, and that is the way of progress on the path of spiritual realization. The king or the executive head of a state, the father and the school teacher are all considered to be natural leaders of the innocent people in general. All such natural leaders have a great responsibility to their dependents; therefore they must be conversant with standard books of moral and spiritual codes.

Commentary by Sri Vishvanatha Chakravarthi Thakur of Gaudiya Sampradaya:

In this verse he explains how the people are taught: by following example.

Commentary by Sri Ramanuja of Sri Sampradaya:

3.21 Whatever ‘an eminent man,’ i.e., he, who is famous for his knowledge of all the scriptures and for his observance of the scriptural dictates, performs, others who have incomplete knowledge of the scriptures will also perform, following his example. With regard to any duty which is being performed with all its ancillaries by an eminent personage, the people with incomplete knowledge will do it with the same ancillaries. Therefore for the protection of the world, all acts that are appropriate to one’s station and stage in life must always be performed by an eminent man who is distinguished for his wisdom. Otherwise, the evil generated from the ruin of the large masses of the world (who neglect their duties by following his example), will bring him down, even if he were a follower of pure Jnana Yoga.

Commentary by Sri Sridhara Swami of Rudra Sampradaya:

How the performance of prescribed Vedic actions acts as an incentive to the masses is what Lord Krishna is emphasising here. That line of reasoning that great personalities accept as authoritative people will also follow.

Commentary by Sri Madhvacharya of Brahma Sampradaya:

The great personalities naturally set the example for the common man to follow in the manner in which they speak and in the way in which they act. This is Lord Krishna’s meaning.

Commentary by Sri Keshava Kashmiri of Kumara Sampradaya:

How Lord Krishna performing actions helps the maintenance of the world is explained here. Whatever actions a great personality performs is accepted by the common people as the path to follow due to their looking up to people in positions of leadership, royalty and fame etc. Why should they think this way when it is a known fact that the Vedic scriptures are the authority on all aspects of life? They think this way because they do not rely on the scriptures they depend upon the actions ot the visibly prominent in society to base their judgments. So whatever a prominent member of society postualtes whether it is Vedic or concocted the ignorant not being knowledgeable will undoubtedly follow correct or not.

Commentary by Sri Adi Shankaracharya of Advaita Sampradaya:

3.21 Yat yat, [This is according to the Ast. The G1. Pr. reads, yat yat yesu yesu.-Tr.] whatever action; a sresthah, superior person, a leader; acarati, does; itarah, another; janah, person, who follows him; does tat tat eva, that very action. Further, yat, whatever; sah, he, the superior person; kurute, upholds; as pramanam, authority, be it Vedic or secular; lokah, an ordinary person; anuvartate, follows; tat, that, i.e. he accepts that very thing as authoritative. ‘If you have a doubt here with regard to the duty of preventing people from straying, then why do you not observe Me?’

Commentary by Sri Abhinavagupta of Kaula Tantra Sampradaya:

3.21 See Comment under 3.22

Sanskrit Shloka Without Transliteration Marks:

yad yad acarati sresthas
tat tad evetaro janah
sa yat pramanam kurute
lokas tad anuvartate

Sanskrit to English Word for Word Meanings:

yat yat — whatever; ācarati — he does; śreṣṭhaḥ — a respectable leader; tat — that; tat — and that alone; eva — certainly; itaraḥ — common; janaḥ — person; saḥ — he; yat — whichever; pramāṇam — example; kurute — does perform; lokaḥ — all the world; tat — that; anuvartate — follows in the footsteps.