This is the 440th most frequent Russian word.
действовать
To act; to operate; to take action.
Here, 'действовать' is employed to describe the action of a law taking effect or becoming operational.
Этот закон начнёт действовать с первого января.
This law will come into effect from the first of January.
In this context, 'действовать' refers to 'functioning' or 'operating,' depicting mechanical or operational activity.
Двигатель перестал действовать из-за поломки.
The engine stopped functioning due to a breakdown.
The word 'действовать' is used here to mean 'to act' or 'to take action'; it conveys engaging in activities toward a goal.