Which are correct:
1 You never know when lightening will strike.
2 You never know when lightening strikes.
3 You never know when your group will be sent on a mission.
4 You never know when your group is sent on a mission.
I think the ones in red are correct. The ones in purple seem to mean that you don't know something is happening (lightening is striking, your group is sent on a mission) when it is happening.
That would work in a sentence like:
You never know when he is picking your pocket. (He is doing it and you are oblivious to it.)
It's "lightning". You used "lightening", which means "to make something more light".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.