4:17  

कर्मणो ह्यपि बोद्धव्यं बोद्धव्यं च विकर्मणः ।
अकर्मणश्च बोद्धव्यं गहना कर्मणो गतिः ॥४-१७॥

karmaNo hyapi boddhawyaM boddhawyaM cha wikarmaNaH ।

akarmaNashcha boddhawyaM gahanaa karmaNo gatiH ॥४-१७॥

Source: Automatic ITRANS Transliteration

Action should be understood, and also prohibited action.
And inaction should be understood. Mysterious are the ways of action.

Source: Milind S. Pandit

The true nature of action is very difficult to understand. Therefore, one should know the nature of attached action, the nature of detached action, and also the nature of forbidden action.

Source: The Bhagavad-Gita: The Sacred Song

The intricacies of action are very hard to understand. Therefore one should know properly what action is, what forbidden action is, and what inaction is.

Source: Bhagavad Gita As It Is