BhG 3.21

syntax

śreṣṭhaḥ (the best) yat yat (something) ācarati (he performs),
itaraḥ (the other) janaḥ (man) tat tat (that) eva [ācarati] (certainly he performs).
saḥ [śreṣṭhaḥ] (he the best) yat (that which) pramāṇam (standard) kurute (he does),
lokaḥ (world) tat (that) anuvartate (it follows).

grammar

yat yat	–	yat sn. 2n.1 n. – that which, something (distributive use);
ācarati	–	ā-√car (to behave, to perform) Praes. P 1v.1 – he performs;
śreṣṭhaḥ	–	śreṣṭha 1n.1 m. – the best, perfect (superlative of: śrī – śreyas, śreṣṭha);
tat tat	–	tat sn. 2n.1 n. – to and to, each respectively (distributive use);
eva	–	av. – certainly, just, merely;
itaraḥ	–	i-tara sn. 1n.1 m. – the other, the rest;
janaḥ	–	jana 1n.1 m. – man, people, creature, race (from: √jan – to be born, to produce);
saḥ	–	tat sn. 1n.1 m. – he;
yat	–	yat sn. 2n.1 n. – that which;
pramāṇam	–	pramāṇa 2n.1 n. – measure, standard, principle (from: pra-√mā – to measure, to make ready);
kurute	–	√kṛ (to do) Praes. Ā 1v.1 – he does;
lokaḥ	–	loka 1n.1 m. – world, people;
tat	–	tat sn. 2n.1 n. – that;
anuvartate	–	anu-√vṛt (to follow, to obey) Praes. Ā 1v.1 – it follows, it obeys;

textual variants

ācarati → ācarate (he performs);

Śāṃkara

The key:
an excerpt from the commented verse
quotes from the scriptures
starting polemic

Who should secure the welfare of the world? And how? The answer follows:		loka-saṃgrahaḥ kim arthaṃ kartavya ity ucyate —
Whatsoever a great man does, that alone the other men do; whatever he sets up as the standard, that the world follows.		yad yad ācarati śreṣṭhas tat tad evetaro janaḥ | sa yat pramāṇaṃ kurute lokas tad anuvartate ||3.21||
Whatever authority the chief among men follows, whether in relation to the spiritual or temporal matters, the same is regarded as the authority by his followers.		yad yat karma ācarati karoti śreṣṭhaḥ pradhānaḥ tat tad eva karma ācarati itaro ’nyaḥ janaḥ tad-anugataḥ | kiṃ ca saḥ śreṣṭhaḥ yat pramāṇaṃ kurute laukikaṃ vaidikaṃ vā lokas tat anuvartate tad eva pramāṇī karoti ity arthaḥ ||3.21||

Rāmānuja

yato jñānayogādhikāriṇo ‚pi karmayoga evātmadarśane śreyān; ata eva hi janakādayo rājarṣayo jñāninām agresarāḥ karmayogenaiva saṃsiddhim āsthitāḥ ātmānaṃ prāptavantaḥ // evaṃ prathamaṃ mumukṣor jñānayogānarhatayā karmayogādhikāriṇaḥ karmayoga eva kārya ity uktvā jñānayogādhikāriṇo ‚pi jñānayogāt karmayoga eva śreyān iti sahetukam uktam / idānīṃ śiṣṭatayā vyapadeśyasya sarvathā karmayoga eva kārya ity ucyate

lokasaṃgrahaṃ paśyann api karmaiva kartum arhasi / śreṣṭhaḥ kṛtsnaśāstrajñātayānuṣṭhātṛtayā ca prathito yad yad ācarati, tat tad evākṛtsnavij jano ‚py ācarati; anuṣṭhīyamānam api karma śreṣṭho yat pramāṇaṃ yadaṅgayuktam anutiṣṭhati tadaṅgayuktam evākṛtsnavilloko ‚py anutiṣṭhati /
ato lokarakṣārthaṃ śiṣṭatayā prathitena śreṣṭhena svavarṇāśramocitaṃ karma sakalaṃ sarvadā anuṣṭheyam; anyathā lokanāśajanitaṃ pāpaṃ jñānayogād apy enaṃ pracyāvayet

Śrīdhara

karma-karaṇe loka-saṅgraho yathā syāt tad āha yad iti | itaraḥ prākṛto ‚pi janas tat tad evācarati | sa śreṣṭho janaḥ karma-śāstraṃ tan-nivṛtti-śāstraṃ vā yat pramāṇaṃ manyate, tad eva loko ‚py anusarati

Madhusūdana

nanu mayā karmaṇi kriyamāṇe ‚pi lokaḥ kim iti tat-saṅgṛhṇīyād ity āśaṅkya śreṣṭhācārānuvidhāyitvād ity āha yad yad iti | śreṣṭhaḥ pradhāna-bhūto rājādir yad yat karmācarati śubham aśubhaṃ vā tat tad evācaratītaraḥ prākṛtas tad-anugato janaḥ | na tv anyat svātantryeṇety arthaḥ |

nanu śāstram avalokyāśāstrīyaṃ śreṣṭhācāraṃ parityajya śāstrīyam eva kuto nācarati loka ity āśaṅkyācāravat pratipattāv api śreṣṭhānusāritām itarasya darśayati sa yad iti | sa śreṣṭho yal laukikaṃ vaidikaṃ vā pramāṇaṃ kurute pramāṇatvena manyate tad eva loko ‚py anuvartate pramāṇaṃ kurute na tu svātantryeṇa kiṃcid ity arthaḥ | tathā ca pradhāna-bhūtena tvayā rājñā loka-saṃrakṣaṇārthaṃ karma kartavyam eva pradhānānuyāyino jana-vyavahārā bhavantīti nyāyād ity abhiprāyaḥ

Viśvanātha

loka-saṅgraha-prakāram evāha yad yad iti

Baladeva

loka-saṅgraha-prakāram evāha yad yad iti | śreṣṭho mahattamo yat karma yathācarati tat karma tathaivetaraḥ kaniṣṭho ‚py ācarati | sa śresṭhas tasmin karmaṇi yac chāstraṃ pramāṇaṃ kurute manyate lokaḥ kaniṣṭho ‚pi tad-anuyāyī tad evānuvartate ‚nasarati | śāstropetaṃ śreṣṭhācaraṇaṃ kalyāṇa-lipsunā kaniṣṭhenānuṣṭheyam ity arthaḥ | itthaṃ ca tejasvinaḥ śreṣṭhasya ca yat kvacit svairācaraṇaṃ tad-vyāvṛtam | tasya śreṣṭha-kṛtatve ‚pi śāstropetatvābhāvāt