Bhagavad Gita 4.17 is Chapter 4, Verse 17, in Jnana Karma Sanyasa Yoga (Path of Knowledge). Below are the Sanskrit text, a transliteration, a plain-English translation and an explanation.
karmaṇo hy api boddhavyaṁ boddhavyaṁ ca vikarmaṇaḥ akarmaṇaś ca boddhavyaṁ gahanā karmaṇo gatiḥ
"You must understand action, forbidden action, and inaction. The nature of action is difficult to understand."
What This Means:
Krishna introduces three categories: karma (right action), vikarma (wrong/forbidden action), and akarma (inaction). All three must be understood. The path of action is deep and subtle, surface judgments are unreliable.
Going Deeper:
'Gahana' (deep, impenetrable) warns us that karma is profound territory. What looks like good action may be vikarma; what looks like inaction may be the highest action. This teaching will culminate in showing that the realized person's action is actually non-action, and the ignorant person's non-action is actually binding action.
How To Apply This:
Don't judge actions superficially, yours or others'. The busy person might be accumulating bad karma through wrong motivation; the seemingly idle person might be doing the deepest work. Evaluate actions by their source (ego vs. wisdom) and motivation (desire vs. dharma), not just their appearance.
Where this verse sits
Chapter 4 has been about knowledge, and from verse 13 Krishna turns it toward action. In 4.16 he says that even the wise are confused about what action and inaction are, and promises to explain. Verse 17 states the three things that must be understood, and verse 18 gives the famous resolution: one who sees inaction in action and action in inaction is wise among men. Verses 19 to 23 then describe the person whose actions, done without desire and with knowledge, leave no residue.
What the words say
Three terms are set out. Karma is action, specifically the action enjoined by scripture and duty; vikarma, on Shankara's reading, is wrong or prohibited action, while Ramanuja takes it as the varied forms that prescribed action takes; akarma is inaction, which Ramanuja identifies with knowledge of the self. Boddhavyam, 'must be understood', is repeated three times, once for each. Gahana, 'deep' or 'impenetrable', describes karmano gatih, the course or true nature of action. Shankara notes that the difficulty is not in listing the categories but in seeing them rightly, because the self is never really an agent, so that the knower's action is in truth inaction, while the ignorant person's stillness, done from motive, is action that binds.
A common misreading
The verse is not recommending inaction, and the chapter closes that reading in 4.18 and again in 3.8, where Krishna says one cannot even maintain the body without acting. Nor, on Shankara's reading, is it telling the reader to stop judging conduct; prohibited action is named so that it can be avoided. Its warning is against surface judgement. A busy life driven by craving may be accumulating exactly the bondage that a quiet, unattached one is shedding. The measure is the knowledge and the motive behind the act, not its visible size.
Key Sanskrit Terms: