BhG 8.18

syntax

ahar-āgame (at the arrival of day) avyaktāt (from the unmaifested) sarvāḥ vyaktayaḥ (all manifestations) prabhavanti (they are born).
rātry-āgame (at the arrival of night) tatra eva (just there) avyakta-saṁjñake (into that named unmanifested) pralīyante (they dissolve).

grammar

avyaktāt	–	a-vyakta (vi-√añj – to decorate, to make visible) PP 5n.1 m. – from the unmanifested, invisible, unevolved;
vyaktayaḥ	–	vyakti 1n.3 f. – manifestations, appearances, distinctness (from: vi-√añj – to decorate, to make visible);
sarvāḥ	–	sarva sn. 1n.3 f. – all;
prabhavanti	–	pra-√bhū (to spring up from, to produce) Praes. P 1v.3 – they are born;
ahar-āgame	–	ahar-āgama 7n.1 m.; TP: ahnar āgama iti – at the arrival of day (from: ahar / ahan – day; ā-√gam – to come, āgama – arrival);
rātry-āgame	–	rātry-āgama 7n.1 m.; TP: rātrer āgama iti – at the arrival of night (from: √rā – to grant or √ram – to play, to rejoice, rātri – night; ā-√gam – to come, āgama – arrival);
pralīyante	–	pra-√lī (to dissolve, to vanish) Praes. Ā 1v.3 – they dissolve;
tatra	–	av. – there (from: tat; indeclinable locative with an ending -tra);
eva	–	av. – certainly, just, merely;
avyakta-saṁjñake	–	avyakta-saṁjñaka 7n.1 m.; yasyāvyakto iti saṁjñāsti – into that named unmanifested (from: vi-√añj – to decorate, to make visible, PP a-vyakta – unmanifested, invisible, unevolved; sam-√jñā – to be of one opinion, to understand, saṁjñā – made known, communicated, called, definied, saṁjñaka – suffix: named);

textual variants

avyaktād vyaktayaḥ sarvāḥ → avyaktā vyaktayaḥ sarvāḥ (all manifested nad unmanifested);
prabhavanty → pralayaṁti (they dissolve);
avyakta-saṁjñake → avyakta-saṁjñike (into that named unmanifested);

Śāṃkara

prajāpater ahani yad bhavati rātrau ca, tad ucyate—

avyaktād avyaktaṃ prajāpateḥ svāpāvasthā | tasmād avyaktād vyaktayo vyajyanta iti vyaktayaḥ sthāvara-jaṅgama-lakṣaṇāḥ sarvāḥ prajāḥ prabhavanty abhivyajyante, ahna āgamo’har-āgamas tasmin ahar-āgame kāle brahmaṇaḥ prabodha-kāle | tathā rātry-āgame brahmaṇaḥ svāpa-kāle pralīyante sarvā vyaktayas tatraiva pūrvokte’vyakta-saṃjñake

Rāmānuja

commentary under the verse BhG 8.19

Śrīdhara

tatra kim ? ata āha avyaktād iti | kāryasyāvyaktaṃ rūpaṃ kāraṇātmakam | tasmād avyaktāt kāraṇa-rūpād vyajyanta iti vyaktayaś carācarāṇi bhūtāni prādurbhavanti | kadā ? ahar-āgame brahmaṇo dinasyopakrame | tathā rātrer āgame brahma-śayane | tasminn evāvyakta-saṃjñake kāraṇa-rūpe pralayaṃ yānti | yad vā te ‚horātra-vida ity etan na vidhīyante | kintu te prasiddhā ahorātra-vido janā brahmaṇo yad ahar vidus tasyāhna āgame ‚vyaktād vyaktayaḥ prabhavanti | yāṃ ca rātriṃ vidus tasyā rātrer āgame pralīyante iti dvayor anvayaḥ

Madhusūdana

yathoktair aho-rātraiḥ pakṣam āsādi-gaṇanayā pūrṇaṃ varṣa-śataṃ prajāpateḥ paramāyur iti kāla-paricchinnatvenānityo ‚sau | tena tal-lokāt punar-āvṛtti-yuktaiva | ye tu tato ‚rvācīnās teṣāṃ tad ahar-mātra-paricchinnatvāt tat-tal-lokebhyaḥ punar-āvṛttir iti kim u vaktavyam ity āha avyaktād iti | atra dainandina-sṛṣṭi-pralayor eva vaktum upakrāntatvāt tatra cākāśādīnāṃ sattvād avyakta-śabdenākhyākṛtāvasthā nocyate | kintu prajāpateḥ svāpāvasthaiva | svāpāvasthaḥ prajāpatir iti yāvat | ahar-āgame prajāpateḥ prabodha-samaye ‚vyaktāt tat-svāpāvasthā-rūpād vyaktayaḥ śarīra-viṣayādi-rūpā bhoga-bhūtayaḥ prabhavanti vyavahāra-kṣamatayā ‚bhivyajyante | rātry-āgame tasya svāpa-kāle pūrvoktāḥ sarvā api vyaktayaḥ pralīyante tiro-bhavanti yata āvirbhūtās tatraivāvyakta-saṃjñake kāraṇe prāg-ukte svāpāvasthe prajāpatau

Viśvanātha

ye tu tato ‚rvācīnās triloka-sthās teṣāṃ tu tasyāhany ahany api pāta ity āha avyaktād iti | atra dainandina-sṛṣṭi-pralayayor ākāśādīnāṃ sattvād avyakta-śabdena svāpāvasthaḥ prajāpatir evocyate iti madhusūdana-sarasvatī-pādāḥ | tataś ca avyaktāt svāpāvasthāt prajāpateḥ sakāśād vyaktayaḥ śarīra-viṣayādi-rūpā bhoga-bhūmayo bhavanti vyavahāra-kṣamāḥ syuḥ | rātry-āgame tasya svāpa-kāle pralīyante tasminn eva tirobhavanti

Baladeva

ye tu tasmād arvācīnās trilokī-vartinas teṣāṃ brahmaṇo dine pātaḥ syād ity āha avyaktād iti | ahar-āgame brahmaṇo jāgara-samaye avyaktāt svāpāvasthāt tasmāt sarvā śarīrendriya-bhogya-bhoga-sthāna-rūpā vyaktayaḥ prabhavanty utpadyante | rātry-āgame tasya svāpa-samaye tatraiva brahmaṇy avyakta-saṃjñake svāpāvasthe kāraṇe tāḥ pralīyante tirobhavanti | atrāvyakta-śabdena pradhānaṃ nābhidheyaṃ dainandina-sṛṣṭi-pralayayor upakramāt | tadā viyad-ādīnāṃ sthitatvāc ca | kintu svāpāvastho brahmaiva tasyārthaḥ