3.1 What is word
Word is a unit of expression that has universal intuitive recognition by native-speakers, whether it is expressed in spoken or written form.词是个表达单位，不管是在口语还是在书面语中，说母语的人对词有种普遍的直觉识别能力。
3.1.1 Three senses of “word”“词”的三种含义
a. a physical definable unit: 是自然的有界限单位
word may be seen as a cluster of sound segments or letters between two pauses or blanks. 词可以看作是两个间隔或空白之间的音段成分或字母的组合群。
b. the common factor underlying a set of forms 是支配一组形式的共同因素。
Word is the common factor underlying a set of forms, a unit of vocabulary, a lexical item, or a lexeme. 词是在一组形式之下的共同要素，是词汇的单位，是一个词条，或是一个词位。
lexicon: a list of all the words in a language assigned to various lexical categories and provided with semantic interpretation.
lexeme(词位):A separate unit of meaning,usually in the form of a word(e.g.”dog in the manger”)
Lexeme-----is postulated as the abstract unit underlying the smallest unit in the lexical system of a language, which appears in different grammatical contexts.词位被假定为语言词汇系统中潜在于最小单位之下的抽象单位，而最小单位是在不同的语法环境中出现的。
e. g. boy boys; check, checks , checking, checked; write, writes, wrote, writing, written; fat, fatter, fattest
boy, check, write, and fat are the lexemes.
c. a grammatical unit 是一个语法单位。
e.g. It is kind of you ,Miss Hou.
Every word plays a grammatical part in the sentence.
3.1.2 Identification of words词的识别
1. stability 稳定性
Words are the most stable of all linguistic units, in respect of their internal structure, that is, the constituent parts of a complex word have little potential for rearrangement, compared with the relative positional mobility of the constituents of sentences in the hierachy.But it is all right for us to rearrange the constituents in a sentence to a certain degree . 所有语言单位中词是最稳定的，就其内部结构来说，跟句子层面成分相对的位置灵活性相比，复合词的组成成分一般不能重新调整次序。但是句子成分却可以做一定程度的重新排列。
e.g. The chairman looked at the audience. 主席看观众
The audience loooked at the chairman. 观众看主席。
2. Relative uninterruptibility 相对的连续性
By uninterruptibility, we mean new elements are not to be inserted into a word even when there are several parts in a word. Nor is allowed to use pauses between the parts of a word. 连续性，是指即使一个词由几个部分构成，新的成分也不能插进词的中间。各部分之间也不允许有停顿。
3. A minimum free form 最小的自由形式
It was first suggested by Leonard Bloomfield. H eadvocated treating sentence as “the maximum free form” and word” the minimum free form”, word being the smallest unit that can constitute, by itself, a complete utterance. 由布龙菲尔德首先提出来。他提倡把句子看作“最大的自由形式”，把词看作“最小的自由形式”。词是能独立构成一个完整语句的最小的单位。
3.1.3 Classification of words 词的分类
a. Variable vs. invariable words 可变化词和不变词
Words can be classfied according to their variability. 根据可变性可以对词进行分类。
In variable words----- one could find ordered and regular series of gramartically different word forms; on the other hand, part of the word remains relatively constant. Thus , each ordered series constitutes paradigm. 关于可变化词，人们可以找到一系列整齐而有规则的词形，它们在语法上是不同的；另一方面，词的一部分相对保持不变。因此每一个整齐的系列构成一个集合。
Invariable words------refer to those words such as since, when, seldom, through, hello. They do not have inflective endings. 不变词指since,when, seldom, through, hello这一类词。它们没有形态结尾。
b. Grammatical words vs. lexical words 语法词和词汇词
In terms of the meaning expressed by words, they can be classified into Grammatical words vs. lexical words.就表达的意义来说，词可以分为语法词和词汇词。
Grammatical words/ function words------those which express grammatical meanings, such as , conjunctions, prepositions, articles, and pronouns, are grammatical words. (表达语法意义的是语法词,如连词,介词,冠词,代词.)
Lexical words/ content words--------those which have lexical meanings, that is , those which refer to substance, action and quality, such as nouns, verbs, adjectives, and adverbs, re lexical words. (具有词汇意义指物质,动作和性质的是词汇词,如名词,动词,形容词,副词.)
As the lexical words carry the main content of a language while the grammatical ones serve to link its different parts together, the lexical words are also known as content words and grammatical ones function words. 词汇词承载了语言的主要内容，而语法词是用来把不同的片段连接在一起的，所以词汇词又叫做实义词，语法词又叫做功能词。
c. Closed-class words vs. open-class words封闭类词和开放类词
The distinction of grammatical words and lexical words leads to distinction of “colsed-class” words and “open-class” words.语法词和词汇词的区别导致了封闭词类和开放词类的区别。
Closed-class word(封闭词类):------ a word whose membership is fixed or limited. New members are not regularly added, such as pronouns, prepositions, conjunctions, articles, etc. .(封闭词类的成员数目是确定的,有限的.新成员不会有规律地增加.例如代词,介词,连词,冠词等.)
open-class word(开放类词):------- a word whose membership is in principle infinite or unlimited,such as nouns,verbs,adjectives,and many adverbs.(开放类词的成员数目原则上是无穷的,无限的.例如名词,动词,形容词和多数副词.)
d. Word class 词类
Classify words either by analyzing the various grammatical, semantic, and phonological properites of the words in language, or by grouping them into classes on the basis of formal similarities in PART OF SPEECH in traditional grammar.要划分词类，更现实的方法是分析词在语言中的不同的语法特征，语义特征和音系特征，或者根据形态变化和分布方面的形式相似形来给词分组。在传统语法中词类part of speech是封闭的。
Particles 、Auxiliaries、 Pro-form 、Determiners 助词，助动词、代词形式、限定词