【正确答案】：
PREFIXES PRIMARILY CHANGE THE MEANING OF THE STEM, WHILE SUFFIXES HAVE ONLY A SMALL SEMANTIC ROLE, THEIR PRIMARY FUNCTION BEING TO CHANGE THE GRAMMATICAL FUNCTION OF STEMS. IN OTHER WORDS, THEY MAINLY CHANGE THE WORD CLASS.
【题目解析】：前缀主要改变词干的意义，而后缀的语义作用很小，其主要功能是改变词干的语法功能。换句话说，它们主要改变单词的词类。