Krishna first introduces the notion of austerity at three levels – bodily, speech, and mental, and then presents the characteristics of austerity in the three modes of nature.

Worship (pūjanaṁ) of the Lord (deva), the twice-born (dvija), the spiritual master (guru), and the wise (prājña), cleanliness (śaucam), simplicity (ārjavam), celibacy (brahmacaryam) and nonviolence (ahiṁsā ca) are said to constitute (ucyate) bodily austerity (śārīraṁ tapa). (BG 17.14)

Speech (vākyaṁ) that is (yat) not causing a disturbance (anudvega-karaṁ), true (satyaṁ), pleasing (priya), and beneficial (hitaṁ ca), as well as (caiva) the practice of studying scriptures (svādhyāyābhyasanaṁ) are said to comprise (ucyate) austerity of the speech (vāṅ-mayaṁ tapa). (BG 17.15)

The satisfaction of the mind (manaḥ-prasādaḥ), gentleness (saumyatvaṁ), silence (maunam), self-control (ātma-vinigrahaḥ) and purity of one’s existence (bhāva-saṁśuddhir) – these (ity etat) are said to constitute (ucyate) mental austerity (tapo mānasam). (BG 17.16)

That (tat) threefold [bodily, speech and mental] (tri-vidhaṁ) austerity (tapas), which is executed (taptaṁ) with transcendental (parayā) faith (śraddhayā) by people (naraiḥ) not seeking any fruits (aphalākāṅkṣibhir) while engaged (yuktaiḥ) is considered to be (paricakṣate) in goodness (sāttvikaṁ). (BG 17.17)

An erratic (calam) and non-permanent (adhruvam) austerity (tapo) that is (yat) performed (kriyate) for the sake of gaining (ārthaṁ) respect (satkāra), prestige (māna) and honor (pūja), and with deceit (dambhena caiva) – that is said to be (tad iha proktaṁ) in the mode of passion (rājasaṁ). (BG 17.18)

An austerity (tapaḥ) that is (yat) performed (kriyate) out of foolishness (mūḍha-grāheṇa) by torturing oneself (ātmano pīḍayā) or (vā) for the sake of (ārthaṁ) destroying others (parasyotsādana), is said to be (udāhṛtam) in the mode of ignorance (tāmasam). (BG 17.19)