【正确答案】正确答案：Affix is a collective term for the type of morpheme that can be used only when added to another morpheme(the root or the stem), so affix is naturally bound. Affixes are generally classified into three subtypes, namely, prefix, suffix, and infix, depending on their position around the root or stem of a word.(3 points)Prefixes such as dis-, mal-, un-, en- can attach productively to verbs and do not change the category. They are all derivational prefixes.(3 points)

【答案解析】解析：考查词缀的定义。词缀是那些只能附加在另一个语素(词根或词干)上的一类语素的总称。因此，词缀本质上是粘着的。根据它们与词根和词干的相对位置，可以分为三类：前缀、后缀和中缀。可以粘着在动词上而不改变其词性的词缀有：dis-，mal-，un-，en-，如dislike，maltreat，unfold，enclose等。