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