Bhagavad Gita Chapter 10, Text 17

Bg 10.17

katham vidyam aham yogims
tvam sada paricintayan
kesu kesu ca bhavesu
cintyo ’si bhagavan maya

Word for word: 
katham — how; vidyam aham — shall I know; yogin — O supreme mystic; tvam — You; sada — always; paricintayan — thinking of; kesu — in which; kesu — in which; ca — also; bhavesu — natures; cintyah asi — You are to be remembered; bhagavan — O Supreme; maya — by me.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O Krishna, O supreme mystic, how shall I constantly think of You, and how shall I know You? In what various forms are You to be remembered, O Supreme Personality of Godhead?

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
As it is stated in the previous chapter, the Supreme Personality of Godhead is covered by His yoga-maya. Only surrendered souls and devotees can see Him. Now Arjuna is convinced that his friend, Krishna, is the Supreme Godhead, but he wants to know the general process by which the all-pervading Lord can be understood by the common man. Common men, including the demons and atheists, cannot know Krishna, because He is guarded by His yoga-maya energy. Again, these questions are asked by Arjuna for their benefit. The superior devotee is concerned not only for his own understanding but for the understanding of all mankind. So Arjuna, out of his mercy, because he is a Vaisnava, a devotee, is opening for the common man the understanding of the all-pervasiveness of the Supreme Lord. He addresses Krishna specifically as yogin because Sri Krishna is the master of the yoga-maya energy, by which He is covered and uncovered to the common man. The common man who has no love for Krishna cannot always think of Krishna; therefore he has to think materially. Arjuna is considering the mode of thinking of the materialistic persons of this world. The words kesu kesu ca bhavesu refer to material nature (the word bhava means “physical things”). Because materialists cannot understand Krishna spiritually, they are advised to concentrate the mind on physical things and try to see how Krishna is manifested by physical representations.