Completed actions (une fois que)

Sequential Actions with "Une fois que"

When defusing a bomb, sequence is everything. To express "once" a specific action is completely finished, French uses une fois que.

To show that an action must be entirely completed before the next one can begin, pair une fois que with a compound tense, such as the passé composé.

Once you have found the bomb, do not touch it.
Une fois que tu as trouvé la bombe, ne la touche pas.

Notice how the completed action (finding the bomb) is in the passé composé, while the subsequent instruction is in the imperative. This strictly orders the events.

Once you have seen the diagram, gently remove the detonator.
Une fois que tu as vu le schéma, retire doucement le détonateur.

You can also use the present tense in the main clause to describe what happens right after a completed step.

Once the rookie has read the diagram, she understands it.
Une fois que la recrue a lu le schéma, elle le comprend.
Once the rookie has defused the bomb, the countdown stops.
Une fois que la recrue a désamorcé la bombe, le compte à rebours s'arrête.