In the last verse, Lord Krsna has briefly answered Arjuna’s question about the difference between sannyāsa and tyāga. Now he makes clear about those karma which are to be given up.
tyājyaṁ doṣavadityeke karma prāhurmanīṣiṇaḥ | yajñadānatapaḥkarma na tyājyamiti cāpare ||18.3||
Some wise men say that action, which is (inherently) defective, is to be given up, and others say that an action, which is a ritual, charity, or religious discipline should not be given up.
prāhurmanīṣiṇaḥ - manīsinah (learned people who have the vision of ātmajñāna) said that karma is to be given up - tyājyaṁ. For whom? For one who only wants moksa. Why? Because doṣavad - inherent with defects. It is considered to have defects because it is the cause of bondage, since every karma produces pāpa or punya and thereby keeps the wheel of samsāra going. This means one should give up karma, take to life of sannyāsa pursuing only knowledge.
But some others say that some karma like yajña - ritual, dāna - giving and tapaḥ - religious discipline are not to be given up - karma na tyājyam. Since these karma can help one gain purity of the mind, therefore for one who is not ready yet, these karma shouldn’t be given up. If out of delusion one gives up these karma, one will be neither a sannyāsī nor a karmī. Lord Krsna presented the choice between giving up all karma as a sannyāsī or giving up only kāmya-karma but do yajña, dāna and tapas as a karma-yogī. Both have to culminate in sarva-karma-sannyāsa which is called vidvat-sannyāsa, the one who knows the self is not an agent or an enjoyer as the result of knowledge.
Lord Krsna wants to make sure that we understand fully what is to be done and what is to be given up.
niścayaṁ śṛṇu me tatra tyāge bharatasattama | tyāgo hi puruṣavyāghra trividhaḥ(s) samprakīrtitaḥ ||18.4||
Arjuna, the most mature among the Bharata family! The tiger among men! Listen to my ascertained opinion about this renunciation. Renunciation is well stated three fold.
About giving up will be explained in 7, 8, and 9 verses later.
Next he explains about what is to be done.
yajñadānatapaḥkarma na tyājyaṁ kāryameva tat | yajño dānaṁ tapaścaiva pāvanāni manīṣiṇām ||18.5||
An action that is a ritual, charity, or religious discipline is not to be given up; that is indeed to be done. Ritual, charity, and religious discipline are indeed purifying for those who are discriminative.
Karma implies a lot of effort, not to talk about ritualistic karma, even ordinary daily chores are not easy. But as a sannyāsī who has no spouse, children, obligation, seems to be the easiest way because one doesn’t need to do anything. But for one who is not ready for it, sannyāsa is not conducive to one’s growth. Therefore certain rituals, giving and religious discipline actually will help a person to grow.
Lord Krsna clarifies in next verse
etānyapi tu karmāṇi saṅgaṁ tyaktvā phalāni ca | kartavyānīti me pārtha niścitaṁ matamuttamam ||18.6|| Even these actions are to be done giving up the attachment and giving up the results. This is my clear proper vision, Pārtha (Arjuna)!
etānyapi tu karmāṇi - these karma yajña, dāna, and tapas are nitya-naimittika-karma which are means for self-purification, but need to be done with the attitude of “they are to be done” - kartavyāni, by seeing it as Īśvara mandate or we even can call it the law / order of nature. Also the attachment toward karma itself and their results is to be given up - saṅgaṁ tyaktvā phalāni ca.
Only with these attitudes, one gets rid of the hold of rāga-dvesa - like and dislike, then karma become means for self-purification. Once one is freed from the hold of rāga-dvesa, one can give up all karma. Then one can totally accept prārabdha-karma.
Next is the explanation from verse 4.
niyatasya tu sannyāsaḥ karmaṇo nopapadyate | mohāt tasya parityāgastāmasaḥ parikīrtitaḥ ||18.7||
Renunciation of enjoined actions is not proper. Renunciation of it (enjoined actions), out of delusion, is called tāmasika.
niyatasya tu sannyāsaḥ - giving up nitya-naimittika-karma is not proper - nopapadyate, if one takes sannyāsa because of delusion - mohāt in the form of some fascination or romanticism about sannyāsa. This kind of giving up is tāmasika - parityāgastāmasaḥ.
duḥkham ityeva yatkarma kāyakleśabhayāt tyajet | sa kṛtvā rājasaṁ tyāgaṁ naiva tyāgaphalaṁ labhet ||18.8||
One may give up the karma as indeed painful out of fear of affliction to one’s physical body. Having done that rājasika renunciation, one would certainly not gain the result of renunciation.
One who gives up mandatory karma because of avoiding the affliction brought about by those karma, this kind of giving up is rājasika which is born of sheer desire. Even though renunciation is done, it wouldn’t help him to get the result of renunciation which is citta-naiścalyam - fixity of the mind only to abide in ātmājñāna.