Determinism is true: everything is caused by something else, and God, being omniscient knows (or would know, if you're an atheist) exactly what is going to happen in every detail from eternity to eternity (quantum indeterminacy aside, but that only applies at the quantum level: at the everyday level, Newtonian physics applies). Therefore it was pre-determined that I would vote Labour at the last general election.
Free-will is true: I was predestined from all eternity to vote labour, but the pre-existing cause of my first of all voting and secondly voting Labour was my free (i.e. unconstrained) choice.
Does that make sense? If not, fire at will, but it would be appreciated if sarcasm could be avoided.