BhG 18.37

syntax

tat-agre (at the beginning of that) yat [sukham] (that  happiness which) viṣam iva [asti] (is like a poison),
pariṇāme (in maturity) [yat sukham] (that  happiness which) amṛtopamam [asti] (is resembling nectar),
tat ātma-buddhi-prasāda-jam (that which results from satisfaction of the self and intelligence) sukham (happiness) sāttvikam (sattvic) proktam (is called).

grammar

yat	–	yat sn. 1n.1 n. – which;
tat-agre	–	av. (7n.1) – at the beginning of that (from: tat sn. – that; agra – foremost, first, summit, goal);
viṣam	–	viṣa 1n.1 n. – active, poison, venom (from: √viṣ – to be active, to perform);
iva	–	av. – like, in the same manner as, almost, exactly;
pariṇāme	–	pariṇāma 7n.1 n. – in a change, in maturity (from: pari-√xnam – to change, to ripen);
amṛtopamam	–	amṛta-upama 1n.1 n.; yasyāmṛtam upameti – resembling nectar (from: √mṛ – to die; mṛta PP – dead, a-mṛta – not dead, eternal, nectar of immortality; upa-√mā – to measure, to compare, upamā – comparison, similarity, upama – suffix: similar, resembling);
tat	–	tat sn. 1n.1 n. – that;
sukham	–	sukha 1n.1 n. – pleasure, comfort (from: su – prefix: good, excellent, beautiful, virtuous; kha – cavity, hole, nave; su-kha – joy, happiness, literally: good hole in the nave [of a wheel through which an axis runs] that makes the moving smooth; or from: su-√sthā; opposite to: duḥkha – pain, difficulty);
sāttvikam	–	sāttvika 1n.1 n. – related to sattva (from: √as – to be, PPr sant – being, existing, true, the essence, abst. sattva – being, essence, wisdom, spirit, one of the three guṇas);
proktam	–	prokta (pra-√vac – to declare, to speak) PP 1n.1 n. – spoken, called;
ātma-buddhi-prasāda-jam	–	ātma-buddhi-prasāda-ja 1n.1 n.; yad ātmano buddheś ca prasādād jāyate tat – which results from satisfaction of the self and intelligence (from: ātman – self; √budh – to wake, to perceive, to understand, buddhi – intelligence, thought, understanding, knowledge, idea, opinion; pra-√sad – to settle down, to be pleased, to be successful, prasāda – satisfaction, favour, calmness, clearness, success, kindness; √jan – to be born; –ja – suffix: born);

textual variants

yat tad agre → yat tadā-tve / tat tad-agre (that which i snow / that at the beginning of that);
proktam → vidyād (he should know);
ātma-buddhi-prasāda-jam → ātma-buddhi-prasādanam (purity of the self and intelligence);

Śāṃkara

yat tat sukham agre pūrvaṃ prathama-saṃnipāte jñāna-vairāgya-dhyāna-samādhy-ārambhe’tyantāyāsa-pūrvakatvāt viṣam iva duḥkhātmakaṃ bhavati, pariṇāme jñāna-vairāgyādi-paripāka-jaṃ sukham amṛtopamam, tat sukhaṃ sāttvikaṃ proktaṃ vidvadbhiḥ | ātmano buddhir ātma-buddhiḥ, ātmabuddheḥ prasādo nairmalyaṃ salilasya iva svacchatā, tato jātam ātma-buddhi-prasādajam | ātma-viṣayā vā ātmāvalambanā vā buddhir ātma-buddhiḥ, tat-prasāda-prakarṣād vā jātam ity etat | tasmāt sāttvikaṃ tat

Rāmānuja

pūrvoktāḥ sarve jñānakarmakartrādayo yac cheṣabhūtāḥ, tac ca sukhaṃ guṇatas trividham idānīṃ śṛṇu /

yasmin sukhe cirakālābhyāsāt krameṇa niratiśayāṃ ratiṃ prāpnoti, duḥkhāntaṃ ca nigacchati nikhilasya sāṃsārikasya duḥkhasyāntaṃ nigacchati // tad eva viśinaṣṭi yat tat sukham, agre yogopakramavelāyāṃ bahvāyāsasādhyatvād viviktasvarūpasyānanubhūtatvāc ca viṣam iva duḥkham iva bhavati, pariṇāme ‚mṛtopamam / pariṇāme vipāke abhyāsabalena viviktātmasvarūpāvirbhāve amṛtopamaṃ bhavati, tac ca ātmabuddhiprasādajam ātmaviṣayā buddhiḥ ātmabuddhiḥ, tasyāḥ nivṛttasakaletaraviṣayatvaṃ prasādaḥ, nivṛttasakaletaraviṣayabuddhyā viviktasvabhāvātmānubhavajanitaṃ sukham amṛtopamaṃ bhavati; tat sukhaṃ sāttvikaṃ proktam

Śrīdhara

idānīṃ sukhasya traividhyaṃ pratijānīte ‚rdhena sukham iti | spaṣṭo ‚rthaḥ ||36|| tatra sāttvikaṃ sukham āha abhyāsād iti sārdhena | yatra yasmiṃś ca sukhe ‚bhyāsād atiparicayād ramate | na tu viṣaya-sukha iva sahasā ratiṃ prāpnoti | yasmin ramamāṇaś ca duḥkhasyāntam avasānaṃ nitarāṃ gacchati prāpnoti | kīdṛśaṃ tat ? yat tat kim apy agre prathamaṃ viṣam iva manaḥ-saṃyamādhīnatvād duḥkhāvaham iva bhavati | pariṇāme tv amṛta-sadṛśam | ātma-viṣayā buddhir ātma-buddhiḥ | tasyāḥ prasādo rajas-tamo-mala-tyāgena svacchatayāvasthānam | tato jātaṃ yat sukhaṃ tat sāttvikaṃ proktaṃ yogibhiḥ

Viśvanātha

sāttvikaṃ sukham āha sārdhena abhyāsāt punar anuśīlanād eva ramate, na tu viṣayeṣv ivotpattyaiva ramata ity arthaḥ | duḥkhāntaṃ nigacchati yasmin ramamāṇaḥ saṃsāra-duḥkhaṃ taratīty arthaḥ | viṣam iveti indriya-mano-nirodho hi prathamaṃ duḥkhada eva bhavatīti bhāvaḥ

Baladeva

atha sukha-traividhyaṃ pratījānīte sukhaṃ tv ity ardhakena | tatra sāttvikaṃ sukham āha abhyāsād iti sārdhakena | abhyāsāt punaḥ punaḥ pariśīlanād yatra ramate, na tu viṣayeṣv ivotpattyā | yasmin ramamāṇo duḥkhāntaṃ nigacchati saṃsāraṃ tarati | yac cāgre prathamaṃ viṣam iva manaḥ-saṃyama-kleśa-sattvād viviktātma-prakāśāc cātiduḥkhāvaham iva bhavati | pariṇāme samādhi-paripāke saty amṛtopamaṃ vivikātma-prakāśāt pīyūṣa-pravāha-nipātavad bhavati | yac cātma-sambandhinyā buddheḥ prasādāj jāyate tat sāttvikaṃ sukham | tat-prasādaś ca viṣaya-sambaandha-mālinya-vinivṛttiḥ