verse 5: tad viddhi

GURU Tattva: Bhagavad Gita 4.34, Sri Krishna to Arjun: 

tad viddhi praṇipātena

paripraśnena sevayā

upadekṣyanti te jñānaṁ

jñāninas tattva-darśinaḥ

Synonyms

tat — that knowledge of different sacrifices; viddhi — try to understand; praṇipātena — by approaching a spiritual master; paripraśnena — by submissive inquiries; sevayā — by the rendering of service; upadekṣyanti — they will initiate; te — you; jñānam — into knowledge; jñāninaḥ — the self-realized; tattva — of the truth; darśinaḥ — seers.

Translation

Just try to learn the truth by approaching a spiritual master. Inquire from him submissively and render service unto him. The self-realized souls can impart knowledge unto you because they have seen the truth.

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Vedabase with Srila ACBVSwami Prabhupad's commentary. 

Srila Bhaktivedanta Narayan Maharaj's commentary is a must read. Page 343 of the PDF, page 273 of the book.

First chapter of Srila Bhakti Rakshak Sridhar Maharaj's Sri Guru and His Grace. 

Very rich beautiful lecture on Guru Tattva expounding this and the other verses delivered by Srila Bhaktivedanta Vamam Maharaj at his 2002 Vyasa Puja. 

An excerpt of Srila Gurudev Srila Bhakti Ballabh Tirtha Maharaj's book Harikatha and Vaishnav Aparadh.