4.37

यथैधांसि समिद्धोऽग्निर्भस्मसात्कुरुतेऽर्जुन ।
ज्ञानाग्निः सर्वकर्माणि भस्मसात्कुरुते तथा ॥

yathaidhāṃsi samiddho 'gnir bhasmasāt kurute 'rjuna
jñānāgniḥ sarvakarmāṇi bhasmasātkurute tathā

Kṛṣṇa (कृष्ण) is still speaking, to Arjuna.

The verse

As a kindled fire makes wood into ash, Arjuna, so the fire of knowledge makes all actions into ash.

नेपालीमा

जसरी बालिएको आगोले काठलाई खरानी बनाउँछ, हे अर्जुन, त्यसरी ज्ञानको आगोले सबै कर्मलाई खरानी बनाउँछ।

Where it sits

4.19 named the man whose actions are burned by the fire of knowledge, and this verse shows the burning: what was done does not stay as a residue to be carried. 4.38 says nothing here cleans as this does.

Split: यथा + एधांसि + समिद्धः + अग्निः + भस्मसात् + कुरुते + अर्जुन + ज्ञान-अग्निः + सर्व-कर्माणि + भस्मसात् + कुरुते + तथा

yathā + edhāṃsi + samiddhaḥ + agniḥ + bhasmasāt + kurute + arjuna + jñāna-agniḥ + sarva-karmāṇi + bhasmasāt + kurute + tathā

Sanskrit, in depth

The words

देवनागरी	Word	Grammar	Sense
यथा	yathā	indeclinable	as.
एधांसि	edhāṃsi	accusative plural	fuels, pieces of wood.
समिद्धः	samiddhaḥ	nominative	kindled, well lit. From सम्-इन्ध् sam-indh.
अग्निः	agniḥ	nominative	fire.
भस्मसात्	bhasmasāt	indeclinable	to ash, into the state of ash. The ending --sāt (सात्) means "into the condition of."
कुरुते	kurute	present, 3rd singular, middle	it makes. From kṛ (कृ).
अर्जुन	arjuna	vocative	Arjuna.
ज्ञान-अग्निः	jñāna-agniḥ	nominative	the fire of knowledge.
सर्व-कर्माणि	sarva-karmāṇi	accusative plural	all actions.
भस्मसात्	bhasmasāt	indeclinable	to ash.
कुरुते	kurute	present, 3rd singular, middle	it makes.
तथा	tathā	indeclinable	so. Answers yathā (यथा).

The phrases

yathā edhāṃsi samiddhaḥ agniḥ bhasmasāt kurute arjuna (यथा एधांसि समिद्धः अग्निः भस्मसात् कुरुते अर्जुन) is the picture. As a kindled fire makes wood into ash. yathā edhāṃsi (यथा एधांसि) is yathaidhāṃsi (यथैधांसि). samiddhaḥ agniḥ (समिद्धः अग्निः) is samiddho 'gniḥ (समिद्धोऽग्निः). agniḥ bhasmasāt (अग्निः भस्मसात्) is agnir bhasmasāt (अग्निर्भस्मसात्). kurute arjuna (कुरुते अर्जुन) is kurute 'rjuna (कुरुतेऽर्जुन).

jñānāgniḥ sarvakarmāṇi bhasmasāt kurute tathā (ज्ञानाग्निः सर्वकर्माणि भस्मसात् कुरुते तथा) is the thing pictured. So the fire of knowledge makes all actions into ash. The same verb, the same bhasmasāt (भस्मसात्), a different fire and a different fuel.

Sanskrit in this line

bhasmasāt (भस्मसात्) is not a case of bhasma (भस्म). The suffix --sāt (सात्) means "into the state of." kurute bhasmasāt (कुरुते भस्मसात्) is "makes into ash." It is used twice, once of wood and once of actions, and tathā (तथा) ties the two uses to yathā (यथा). If you decline bhasmasāt (भस्मसात्) as a noun, the simile loses the "into."

Say it back

What the wood becomes, and what the actions become. If bhasmasāt (भस्मसात्) is said only once, the simile is half gone.