In plain words
How may I know You, O Yogin, by continually contemplating You? In what various forms and aspects should I think of You, Bhagavan?
Word for word
How may I know You, O Yogin, always contemplating [You]? And in which and which things are You to be contemplated by me, O Bhagavān?
The commentators
Bhagavadgītābhāṣyam· bhāṣya
कथं विद्यां विजानीयाम् अहं हे योगिन् त्वां सदा परिचिन्तयन् । केषु केषु च भावेषु वस्तुषु चिन्त्यः असि ध्येयः असि भगवन् मया ॥ १७ ॥
In plain words
How am I to know you — to really discern you — yogin, holding you in mind without a break? And in which states, which actual things, one by one, are you to be thought of, to be meditated upon by me, Bhagavan?
Close to the Sanskrit
How may I know — may I fully discern — I, O yogin, you, always dwelling on you in thought? And in which, in which states — that is, in which things — are you to be thought of, are you to be meditated upon, O Bhagavan, by me?