BhG 18.64

syntax

[tvam] (you) me (my) sarva-guhya-tamam (the most secret of all) paramam vacaḥ (supreme word) bhūyaḥ (again) śṛṇu (you must listen).
[tvam] (you) me (my) dṛḍham (firmly) iṣṭaḥ (beloved) asi (you are),
tataḥ (therefore) [aham] (I) te (to you) hitam (benefit) vakṣyāmi (I will declare).

grammar

sarva-guhyatamam	–	sarva-guhyatama 2n.1 n.; TP: sarvebhyaḥ guhya-tamam iti – the most secret of all (from: sarva – all, whole; √xguh – to cover, to hide, PF guhya – to be hidden, secret, mystery; superlative of: guhya – guhya-tara, guhya-tama);
bhūyaḥ	–	av. – more, again, besides;
śṛṇu	–	√śru (to hear, to listen) Imperat. P 2v.1 – you must listen;
me	–	asmat sn. 6n.1 – my (shortened form of: mama);
paramam	–	parama 2n.1 n. – supreme, the highest (superlative of: para – beyond, ancient, final, the best, the supreme);
vacaḥ	–	vacas 2n.1 n. – word, the speech (from: √vac – to speak);
iṣṭaḥ	–	iṣṭa (from: √iṣ – to desire or √yaj – to consecrate, to sacrifice, to worship) PP 1n.1 m. – desired, worshipped, beloved;
asi	–	√as (to be) Praes. P 2v.1 – you are;
me	–	asmat sn. 6n.1 – my (shortened form of: mama);
dṛḍham	–	av. – firmly, thoroughly, very well (from: √dṛṁh – to make strong, PP dṛḍha – firm, hard, not to be bent);
iti	–	av. – thus (used to close the quotation);
tataḥ	–	av. – then, after that, from that, for that reason (from: tat – indeclinable ablative with an ending -tas);
vakṣyāmi	–	√vac (to declare) Fut. P 3v.1 – I will declare;
te	–	yuṣmat sn. 4n.1 – to you (shortened form of: tubhyam);
hitam	–	hita (√dhā – to put) PP 2n.1 m. – set, beneficial; welfare, benefit;

textual variants

iṣṭo ‘si → dṛṣṭo ‘si (you are seen);
dṛḍham iti → dṛḍha-matiḥ / dṛḍha-mataṁ / dṛḍha iti (who has firm conviction / firm opinion / firm);

The second pada of verse 18.64 is the same as the second pada of verse BhG 10.1;

Śāṃkara

bhūyo’pi mayocyamānaṃ sṛṇu—
sarva-guhyatamaṃ sarvebhyo guhyebhyo’tyanta-guhyatamam atyanta-rahasyam, uktam apy asakṛd bhūyaḥ punaḥ sṛṇu me mama paramaṃ prakṛṣṭaṃ vaco vākyam | na bhayān nāpy artha-kāraṇād vā vakṣyāmi | kiṃ tarhi ? iṣṭaḥ priyo’si me mama dṛḍham avyabhicāreṇeti kṛtvā tatas tena kāraṇena vakṣyāmi kathayiṣyāmi te tava hitaṃ paramaṃ jñāna-prāpti-sādhanam | tad dhi sarva-hitānāṃ hitatamam

Rāmānuja

sarveṣv eteṣu guhyeṣu bhaktiyogasya śraiṣṭhyād guhyatamam iti pūrvam evoktam „idaṃ tu te guhyatamaṃ pravakṣyāmy anasūyave” ityādau / bhūyo ‚pi tadviṣayaṃ paramaṃ me vacaḥ śṛṇu / iṣṭo ‚si me dṛḍham iti tatas te hitaṃ vakṣyāmi

Śrīdhara

ati-gambhīraṃ gītā-śāstram aśeṣataḥ paryālocayitum aśaknuvataḥ kṛpayā svayam eva tasya sāraṃ saṅgṛhya kathayati sarva-guhyatamam iti tribhiḥ | sarvebhyo ‚pu guhyebhyo guhyatamaṃ me vacas tatra tatroktam api bhūyaḥ punar api vakṣyamaṇaṃ śṛṇu | punaḥ punaḥ kathane hetum āha dṛḍham atyantaṃ me mama tvam iṣṭaḥ priyo ‚sīti matvā tata eva hetos te hitaṃ vakṣyāmi | yad vā tvaṃ mameṣṭo ‚si mayā vakṣyamāṇaṃ ca dṛḍhaṃ sarva-pramāṇopetam iti niścitya tatas te vakṣyāmīty arthaḥ | dṛḍha-matir iti kecit paṭhanti

Viśvanātha

tataś ca atigambhīrārthaṃ gītā-śāstraṃ paryālocayitum pravartamānaṃ tuṣṇīmbhūyaiva sthitaṃ sva-priya-sakham arjunam ālakṣya kṛpā-dravac-citta-nava-nīto bhagavān bhoḥ priyavayasya arjuna sarva-śāstra-sāram aham eva ślokāṣṭakena bravīmi | alaṃ te tat-tat-paryālocana-kleśenety āha sarveti | bhūya iti rāja-vidyā-rāja-guhyādhyāyānte pūrvam uktam |
man-manā bhava mad-bhakto
mad-yājī māṃ namaskuru |
mām evaiṣyasi yuktvaivam
ātmānaṃ mat-parāyaṇaḥ || (Gītā 9.34) iti
yat tad eva paramaṃ sarva-śāstrārtha-sārasya gītā-śāstrasyāpi sāraṃ guhyatamam iti nātaḥ paraṃ kiñcana guhyam asti kvacit kutaścit katham api akhaṇḍam iti bhāvaḥ | punaḥ kathane hetum āha | iṣṭo ‚si me dṛḍham atiśayena eva priyo me sakhā bhavasīti tata eva hetor hitaṃ te iti sakhāyaṃ vinātirahasyaṃ na kam api kaścid api brūta iti bhāvaḥ | dṛḍha-matir iti ca pāṭhaḥ

Baladeva

atha nirapekṣāṇāṃ sādhana-sādhya-paddhatim upadeksyann ādau tāṃ stauti sarveti | sarveṣu guhyeṣu madhye ‚tiśayitaṃ guhyam iti sarva-guhyatamam | bhūya iti rāja-vidyādhyāye man-manā bhava ity ādinā pūrvam api mamātipriyatvād ante punar ucyamānaṃ śṛṇu paramaṃ sarva-sārasyāpi gītā-śāstrasya sārabhūtam | punaḥ kathanena hetuḥ iṣṭo ‚sīti | tvaṃ mameṣṭaḥ priyatamo ‚si | mad-vākyaṃ dṛḍhaṃ nikhila-pramāṇopetam iti niścinoṣy atas te hitaṃ vakṣyāmi | tathāpy etad evānuṣṭheyam iti bhāvaḥ