BhG 18.31

syntax

he pārtha (O son of Pṛthā!),
yayā [buddhyā] (that intelligence by which) [puruṣaḥ] (a person) dharmam adharmam ca (the law and lawlessness) kāryam akāryam eva ca (duty and improper activity) ayathāvat (improperly) prajānāti (he understands),
sā buddhiḥ (that intelligence) rājasī [asti] (is rajasic).

grammar

yayā	–	yat sn. 3n.1 f. – that by which;
dharmam	–	dharma 2n.1 m. – the law (from: √dhṛ – to hold);
adharmam	–	adharma 2n.1 m. – lawlessness (from: √dhṛ – to hold);
ca	–	av. – and;
kāryam	–	kārya (√kṛ – to do) PF 2n.1 n. – to be done, work, duty, especially religious one;
ca	–	av. – and;
akāryam	–	akārya (√kṛ – to do) PF 2n.1 n. – not to be done, improper activity;
eva	–	av. – certainly, just, merely;
ca	–	av. – and;
ayathāvat	–	av. – improperly, incorrectly (do: yathā – as; -vat – suffix: as, like);
prajānāti	–	pra-√jñā (to understand) Praes. P 1v.1 – he understands;
buddhiḥ	–	buddhi 1n.1 f. – intelligence, thought, understanding, knowledge, idea, opinion (from: √budh – to wake, to perceive, to understand);
sā	–	tat sn. 1n.1 f. – that;
pārtha	–	pārtha 8n.1 m. – O son of Pṛthā (from: √pṛth – to extend, pṛthā – Kuntī, mother of the sons of Pāṇḍu);
rājasī	–	rājasī 1n.1 f. – pertaining to rajas, rajasic (from: √rañj – to be dyed, be excited, be delighted, rajas – coloured, dust, passion, one of the three guṇas);

textual variants

yayā → yathā (as);
cākāryam → vākāryam (or improper activity);
ayathāvat → atha yāvat (thus as);
ayathāvat prajānāti → yathā-van nābhijānāti (he does not understand properly);

Śāṃkara

yayā dharmaṃ śāstra-coditam adharmaṃ ca tat-pratiṣiddhaṃ kāryaṃ cākāryam eva ca pūrvokte eva kāryākārye ayathāvan na yathāvat sarvato nirṇayena na prajānāti, buddhiḥ sā pārtha rājasī

Rāmānuja

yathā pūrvoktaṃ dvividhaṃ dharmaṃ tadviparītaṃ ca tanniṣṭhānāṃ deśakālāvasthādiṣu kāryaṃ cākāryaṃ ca yathāvan na jānāti, sā rājasī buddhiḥ

Śrīdhara

rājasīṃ buddhim āha yayeti | ayathāvat sandehāspadatvenety arthaḥ | spaṣṭam anyat

Viśvanātha

ayathāvat asmayaktayety arthaḥ

Baladeva

rājasīṃ buddhim āha yayeti | ayathāvad asamyatvena