system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] So, I am an 18 year old lad living on a moderately well to do world. What kind of knowledge would I have about the empire and FTL propulsion?
gpt
You know that your world is part of a galactic Empire, that Holy Terra is its center and guiding light, but that your world (like all worlds) has its unique and irreplaceable role to play in the Emperor's plan.

You know that your world is routinely visited by the servants of the Administratum, whose mighty ships traverse the depthless void of space to collect the tithe your planet owes to the Imperium in return for its protection.  You know, too, that space is vast and full of horrors, but that the full might of the noble Imperial Guard, whose swift ships fearlessly fly from star to star, stands ready to support your Planetary Defense Force should it encounter a threat beyond its capabilities.  If you are high in the ranks of your planet's social structure, you may even know of the Astronomicon, how it holds the Empire together, and of the dread Black Ships which carry psykers to Holy Terra to fuel its beacon.

Will you know about the Warp and what it wants to do to you?  Or about how dangerous FTL travel is?  Not unless you work your way into a position where you need to know.  Knowledge of Chaos by the unauthorized is punishable by death - and for very good reasons.  But even on the most backwater of the Imperium's planets, people know of the Emperor and of Holy Terra, and that His servants travel from star to star in His name.  Space travel is no secret.