13.21

कार्यकारणकर्तृत्वे हेतुः प्रकृतिरुच्यते ।
पुरुषः सुखदुःखानां भोक्तृत्वे हेतुरुच्यते ॥

kāryakāraṇakartṛtve hetuḥ prakṛtir ucyate
puruṣaḥ sukhaduḥkhānāṃ bhoktṛtve hetur ucyate

Kṛṣṇa (कृष्ण) is still speaking, to Arjuna (अर्जुन).

The verse

Nature is called the cause in the doership of effect and instrument; the person is called the cause in the experiencing of pleasure and pain.

नेपालीमा

गरिने काम र गर्ने साधनको कर्ता हुनुमा प्रकृतिलाई कारण भनिन्छ, सुख र दुःख भोग्नुमा पुरुषलाई कारण भनिन्छ।

Where it sits

13.20 said both are beginningless and that the changes and the guṇa (गुण)s come from nature, and this verse splits the work, doing to nature and the experiencing of pleasure and pain to the person.

Split: कार्य-कारण-कर्तृत्वे + हेतुः + प्रकृतिः + उच्यते + पुरुषः + सुख-दुःखानाम् + भोक्तृत्वे + हेतुः + उच्यते

kārya-kāraṇa-kartṛtve + hetuḥ + prakṛtiḥ + ucyate + puruṣaḥ + sukha-duḥkhānām + bhoktṛtve + hetuḥ + ucyate

Sanskrit, in depth

The words

देवनागरीWordGrammarSense
कार्य-कारण-कर्तृत्वेkārya-kāraṇa-kartṛtvelocative singularin the doership of effect and instrument. kārya (कार्य) is what is produced, the body as the work. kāraṇa (कारण) is the means, the senses as instruments. kartṛtva (कर्तृत्व) is the state of being the doer.
हेतुःhetuḥnominative singularthe cause.
प्रकृतिःprakṛtiḥnominative singularnature. Recited prakṛtir (प्रकृतिर्).
उच्यतेucyatepresent passive, 3rd singularis called.
पुरुषःpuruṣaḥnominative singularthe person.
सुख-दुःखानाम्sukha-duḥkhānāmgenitive pluralof pleasures and pains.
भोक्तृत्वेbhoktṛtvelocative singularin the state of being the experiencer. bhoktṛ (भोक्तृ) is the one who experiences.
हेतुःhetuḥnominative singularthe cause. Recited hetur (हेतुर्).
उच्यतेucyatepresent passive, 3rd singularis called.

The phrases

kāryakāraṇakartṛtve hetuḥ prakṛtiḥ ucyate (कार्यकारणकर्तृत्वे हेतुः प्रकृतिः उच्यते) gives nature her job. In the doing of effect and instrument, nature is called the cause. The locative answers "in what role."

puruṣaḥ sukhaduḥkhānāṃ bhoktṛtve hetuḥ ucyate (पुरुषः सुखदुःखानां भोक्तृत्वे हेतुः उच्यते) gives the person his job. In the experiencing of pleasure and pain, the person is called the cause. The genitive sukhaduḥkhānām (सुखदुःखानाम्) answers "of what."

Sanskrit in this line

प्रकृतिः + उच्यते prakṛtiḥ + ucyate is prakṛtir ucyate (प्रकृतिरुच्यते). हेतुः + उच्यते hetuḥ + ucyate is hetur ucyate (हेतुरुच्यते). Visarga after i (इ), and visarga after u (उ), before a vowel, both become r (र्). The two lines of the verse are the same change on two vowels.

The locative --tve (त्वे) is the abstract --tva (त्व) in the case that answers "in." kartṛtve (कर्तृत्वे) is "in doership." bhoktṛtve (भोक्तृत्वे) is "in experiencership."

Say it back

Give nature her cause, and the person his. If the person comes out as the doer of the acts, stay here.