Sub- means under, below
subconscious (adj) - not fully aware; occurring below your level of thinking and awareness
subcutaneous (adj) - under the skin
subdue (v) - to bring under control
subfreezing (adj) - below the freezing point
subirrigate (v) - to water something from under the ground
subject (v) - to put under someone else's control
submarine (n) - a watercraft that stays under water
submerge (v) - to put under water
subordinate (n) - a person who is under someone else in rank or importance
subway (n) - a passageway under the ground; an electric underground railway