A ROOT is the basic form of a word that cannot further be analyzed without total loss of identity.That is to say,it is that part of word left when all the affixes are removed.In the word internationalism,after the removal of inter-,-al and -ism,what is left is the root nation.All words contain a root morpheme.An AFFIX is the collective term for the type of formative that can be used only when added to another morpheme(the root or stem).Affixes are limited in number in a language,and are generally classified into three subtypes,namely,prefix,suffix,and infix,depending on their position reference to the root or stem of the word.A root may be or bound,but an affix is naturally bound.Free root morphemes are those that can stand by themselves and are the base forms of words,such as black in black ,blackbird,blackboard,blacksmith.Morphemes of this type are potentially unlimited in number in a language.However,there are relatively few bound root morphemes in English,such as -ceive in receive,perceive,and conceive;-mit in remit,permit,commit,and submit;-tain in retain,contain,and maintain;-cur in incur,recur,and occur,etc.A few english roots may have both free and bound variants.For instance,sleep and child are free root morphemes,whereas slep- in the former's past tense form slept and child- in the latter's plural form children cannot exist by themselves,and are therefore bound.A stem is any morpheme or combination of morphemes to which an inflectional affix can be added.friend- in friends,and friendship in friendships are both stems.wheares the latter shows that a stem may contain a rot and a derivational affix.

词根是词的基本形式，不能再作进一步的分析而完全不损失同一性。也就是说，去掉所有的词缀后，词所剩下来的部分就是词根。internationalism这个词中，去掉inter-,-al,-ism之后，剩下来的就是词根nation.所有的词都包含一个词根语素。词缀是一个构词成分的集合，它们只能附加于另一个词素（词根或词干）上。语言中词缀的数量总是有限的，根据它们跟词根或词干的相对位置，一般可以把词缀分为三小类，就是前缀，后缀和中缀。词根可能是自由语素也可能是粘附语素，但词缀都是粘附语素。自由词根语素能独自出现，是词的基础形式，如black,blackbird,blackboard,blacksmith中的black.语言中这类语素在数量上的潜力是无限的。可是英语中的粘附词根语素，相对来说数量很少，如receive,perceive,conceive中的-ceive,remit,permit,commit,submit中的-mit,retain,contain,maintain中的-tain,incur,recur,occur中的-cur等。英语中有些词根既可以是自由的，也可以是粘附的。例如，sleep和child都是自由词根语素，然而sleep的过去分词形式slept中的slep-和child的复数形式children中的child-都不能独自存在，因此是粘附的。词干是指能加上屈折词缀的语素或语素的组合。friends中的friend-，friendships中的friendship-都是词干。friend-表明词干可能相当于词根，而friendship-表明词干可以包含词根和派生词缀。以上是《语言学教程》对词干，词根和词缀方面的解释，可能与原文略有不符，只是对其主要内容的归纳，如果想了解更多内容，诸如关于词缀，词干，词根的定义，以及它们之间的联系与区别，可以查阅其他相关资料。