कर्मणो ह्यपि बोद्धव्यं बोद्धव्यं च विकर्मणः।अकर्मणश्च बोद्धव्यं गहना कर्मणो गतिः॥
karmaṇo hyapi boddhavyaṃ boddhavyaṃ ca vikarmaṇaḥakarmaṇaś ca boddhavyaṃ gahanā karmaṇo gatiḥ
Word by word
The verse says
You have to understand action, and you have to understand wrong action, and you have to understand not acting. The way action works runs deep.
In plain words
Three things to understand, and then a warning.
Karma - action. Vikarma - action gone wrong. Akarma - not acting.
And gahanā karmaṇo gatiḥ - the course of action is deep, hard to fathom. Not complicated. Deep.
Where you've felt this · at work
A decision that felt small at the time turns out, five years later, to have been the one that mattered. And the one you agonised over for a month changed nothing at all. On the day, you could not have told the difference.
Going deeper
Gahana means deep, dense, impenetrable - the word used for thick forest. It is the same image as the thicket at 2.52.
What makes action's course hard to trace is that consequences do not stay where you put them. They travel, combine with other people's, and surface somewhere you were not looking.
The three categories are worth separating. Action that fits. Vikarma, usually read as action that is forbidden or distorted - the same energy pointed wrong. And akarma, the absence of action, which Chapter 3 has already shown to be mostly unavailable.
The Gita's answer over the next verses is not a better set of categories. It is a change of test: stop classifying by what the body did, and start classifying by whether anything was grasped.
Which is why verse 18 sounds like a riddle. It is the new rule, stated in the vocabulary of the old one.