Sub- is used at the beginning of words that have `under' as part of their meaning.
Sub- is added to the beginning of nouns in order to form other nouns that refer to things that are part of a larger thing.
Sub- is added to the beginning of adjectives in order to form other adjectives that describe someone or something as inferior, for example inferior to normal people or to normal things.
substitute: be a substitute; "The young teacher had to substitute for the sick colleague"; "The skim milk substitutes for cream--we are on a strict diet"