How to use for as a conjunction [closed]

When I recall the short way of remembering a list of the conjunctions, FANBOYS, for, and, nor, but, or, yet, so, how could for be used as a conjunction? Could it be used in

Justin and Mark could not see, for they were blind.

Solution 1:

Justin and Mark could not see, for they were blind.

The sentence makes sense and is correct. There's a saying "can't see the forest for the trees" which also uses for in this way. However, this construction is formal sounding and maybe a little old fashioned. Most people would say:

Justin and Mark could not see because they were blind.