BhG 3.8

syntax

tvam (you) niyatam (regular) karma (activity) kuru (do),
akarmaṇaḥ hi (than inactivity) karma (activity) jyāyaḥ (better),
akarmaṇaḥ ca (and from inactivity) te (your) śarīra-yātrā api (even the journey of the body) na prasidhyet (it would not succeed).

grammar

niyatam	–	niyata (ni-√yam – to hold back, to regulate) PP 2n.1 n. – regular, steady; or av. – constantly, decidedly;
kuru	–	√kṛ (to do) Imperat. P 2v.1 – do;
karma	–	karman 2n.1 n. – activity (from: √kṛ – to do);
tvam	–	yuṣmat sn. 1n.1 – you;
karma	–	karman 1n.1 n. – activity (from: √kṛ – to do);
jyāyaḥ	–	jyāyaḥ 1n.1 n. – better, higher (comparative of √ji – to conquer, jaya : jyāyas, jyeṣṭha);
hi	–	av. – because, just, indeed, surely;
akarmaṇaḥ	–	a-karman 5n.1 n. – than inactivity (from: √kṛ – to do);
śarīra-yātrā	–	śarīra-yātrā 1n.1 f.; TP: śarīrasya / śarīre yātrā – journey of the body / in the body (from: √śṝ – to break, to crush, śarīra – easy to be destroyed, the body; √yā – to go, yātrā – journey, way, maintenance);
api	–	av. – although, moreover, besides, even;
ca	–	av. – and;
te	–	yuṣmat sn. 6n.1 – your (shortened form of: tava);
na	–	av. – not;
prasidhyet	–	pra-√sidh (to succeed) Pot. P 1v.1 – it would succeed;
akarmaṇaḥ	–	a-karman 5n.1 n. – than inactivity (from: √kṛ – to do);

Śāṃkara

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

Wherefore:		yataḥ evam ataḥ –
Do thou perform (thy) bounden duty; for, action is superior to inaction. And even the maintenance of the body would not be possible for thee by inaction.		niyataṃ kuru karma tvaṃ karma jyāyo hy akarmaṇaḥ | śarīrayātrāpi ca te na prasidhyed akarmaṇaḥ ||3.8||
Thy bounden duty is the obligatory (nitya) act, that which one is bound to perform, and which is not prescribed (in the scriptures) as a means to a specific end. Action is superior to inaction in point of result.		niyataṃ nityaṃ śāstropadiṣṭam | yo yasmin karmaṇy adhikṛtaḥ phalāya cāśrutaṃ tan niyataṃ karma, tat kuru tvaṃ he arjuna ! yataḥ karma jyāyo ’dhikataraṃ phalataḥ | hi yasmād akarmaṇo ’karaṇāt anārambhāt |
By inaction you cannot attain success in the life’s journey. The distinction between action and inaction is thus seen in our own experience.		kathaṃ? śarīra-yātrā śarīra-sthitir api ca te tava na prasidhyet prasiddhiṃ na gacchet akarmaṇo ’karaṇāt | ato dṛṣṭaḥ karmākarmaṇor viśeṣo loke ||3.8||

Rāmānuja

niyataṃ vyāptam; prakṛtisaṃsṛṣṭena hi vyāptaṃ karma, anādivāsanayā prakṛtisaṃsṛṣṭas tvaṃ niyatatvena suśakatvād asaṃbhāvitapramādatvāc ca karmaṇaḥ, karmaiva kuru; akarmaṇaḥ jñānaniṣṭhāyā api karmaiva jyāyaḥ / „naiṣkarmyaṃ puruṣo ‚śunute” iti prakramād akarmaśabdena jñānaniṣṭhaivocyate / jñānaniṣṭhādhikāriṇo ‚py anabhyastapūrvatayā hy aniyatatvena duḥśakatvāt sapramādatvāc ca jñānaniṣṭhāyāḥ, karmaniṣṭhaiva jyāyasī; karmaṇi kriyamāṇe ca ātmayāthātmyajñānenātmano ‚kartṛtvānusandhānam anantaram eva vakṣyate / ata ātmajñānasyāpi karmayogāntargatatvāt sa eva jyāyān ityarthaḥ / karmaṇo jñānaniṣṭhāyā jyāyastvavacanaṃ jñānaniṣṭhāyām adhikāre saty evopapadyate /
yadi sarvaṃ karma parityajya kevalaṃ jñānaniṣṭhāyām adhikāro ‚pi, tarhi akarmaṇaḥ jñānaniṣṭhasya jñānaniṣṭhopakāriṇī śarīrayātrāpi na setsyati / yāvat sādhanasamāpti śarīradhāraṇaṃ cāvaśyaṃ kāryam / nyāyārjitadhanena mahāyajñādikaṃ kṛtvā tacchiṣṭāśanenaiva śarīradhāraṇaṃ kāryam, „āhāraśuddhau sattvaśuddhiḥ sattvaśuddhau dhrutvā smṛtiḥ” ityādiśruteḥ / „te tv aghaṃ bhuñjate pāpā ye pacanty ātmakāraṇāt” iti vakṣyate / ato jñānaniṣṭhasyāpi karmākurvato dehayātrāpi na setsyati / yato jñānaniṣṭhasyāpi dhriyamāṇaśarīrasya yāvatsādhanasamāpti mahāyajñādi nityanaimittikaṃ karma avaśyaṃ kartavyam, yataś ca karmayoge ‚py ātmano ‚kartṛtvabhāvanayātmayāthātmyānusandhānam antarbhūtam, yataś ca prakṛtisaṃsṛṣṭasya karmayogaḥ suśako ‚pramādaś ca, ato jñānaniṣṭhāyogyasyāpi jñānayogāt karmayogo jyāyān / tasmāt tvaṃ karmayogam eva kurv ityabhiprāya

Śrīdhara

niyatam iti | yasmād evaṃ tasmān niyataṃ nityaṃ karma sandhyopāsanādi kuru | hi yasmāt | sarva-karmaṇo ‚karaṇāt sakāśāt karma-karaṇaṃ jyāyo ‚dhikataram | anyathākarmaṇaḥ sarva-karma-śūnyasya tava śarīra-yātrā śarīra-nirvāho ‚pi na prasidhyen na bhavet

Madhusūdana

yasmād evaṃ tasmān manasā jñānendriyāṇi nigṛhya karmendriyais tvaṃ prāg ananuṣṭhita-śuddhi-hetu-karmā niyataṃ vidhy-uddeśe phala-sambandha-śūnyatayā niyata-nimittena vihitaṃ karma śrautaṃ smārtaṃ ca nityam iti prasiddhaṃ kuru | kurv iti madhyama-puruṣa-prayogeṇaiva tvam iti labdhe tvam iti padam arthāntare saṃkramitam |

kasmād aśuddhāntaḥ-karaṇena karmaiva kartavyaṃ hi yasmād akarmaṇo ‚karaṇāt karmaiva jyāyaḥ praśasyataram | na kevalaṃ karmābhāve tavāntaḥ-karaṇa-śuddhir eva na sidhyet | kintu akarmaṇo yuddhādi-karma-rahitasya te tava śarīra-yātrā śarīra-sthitir api na prakarṣeṇa kṣātra-vṛtti-kṛtatva-lakṣaṇena sidhyet | tathā ca prāg uktam | api cety antaḥ-karaṇa-śuddhi-samuccayārthaḥ

Viśvanātha

tasmāt tvaṃ niyataṃ nityaṃ sandhyopāsanādi# akarmaṇaḥ karma-sannyāsāt sakāśāj jyāyaḥ śreṣṭham | sannyāsa-sarva-karmaṇas tava śarīra-nirvāho ‚pi na sidhyet

Baladeva

niyatam iti tasmāt tvam aviśuddha-citto niyatam āvaśyaka-karam kuru citta-viśuddhaye niṣkāmatayā sva-vihitaṃ karmācarety arthaḥ | akarmaṇam autsukya-mātreṇa sarva-karma-saṃnyāsa-sakāśāt karmaiva jyāyaḥ praśastataraṃ krama-sopāna-nyāyena jñānotpādakatvāt | autsukya-mātreṇa karma tyajator maline hṛdi jñāna-prakāśāt | kiṃ cākarmaṇaḥ saṃnyasta-sarva-karmaṇas tava śarīra-yātrā deha-nirvāho ‚pi na sidhyet | yāvat sādhana-pūrti-deha-dhāraṇasyāvaśyakatvāt tad-arthaṃ jñānī bhikṣāṭanādi-karmānutiṣṭhati | tac ca kṣatriyasya tavānucitam | tasmāt sva-vihitena yuddha-prajā-pālanādi-karmaṇā śulkāni vittāny upārjya tair nirvyūha-deha-yātraḥ svātmānam anusandhehīti