Which prefix means "under," "beneath," or "below"?

The prefix that means "under," "beneath," or "below" is "sub." This prefix is derived from Latin, where it is commonly used in various words across different contexts in English. For instance, in terms like "submarine" (underwater) and "substandard" (below the acceptable standard), "sub" effectively conveys the idea of being beneath or lower than something in a particular context.

The other prefixes have different meanings; "dys" generally pertains to difficulty or abnormality (as seen in words like "dysfunctional"), "tachy" refers to speed (as in "tachycardia," which indicates an abnormally fast heart rate), and "cost" relates to rib or rib-like structures (as in "costal"). Understanding the meanings of these prefixes is essential for deciphering medical terminology and other complex words.