One is -sub
An example if a word with the directional prefix -sub is submarine
How it changes the root word (marine) is, marine means: of, found in, or produced by the sea .
Submarine means:a warship with a streamlined hull designed to operate completely submerged in the sea for long periods, equipped with an internal store of air and a periscope and typically armed with torpedoes and/or missiles
So marine has to do with a person, while submarine has to do with a warship
Hope it helped