句子功能是句子所特有的句法范畴，也是语法研究中的重要概念之一，学术界对句子功能的研究多从语用、语义等层面出发，是一种舍语法而就其他的思路。本书按照句子功能研究应当回归句法层而的思想，重新确定了句子功能的性质和范围，认为句子功能是作用于全句的非线性特征，它们必须通过移位、加标、重叠、删除等有限的句法操作手段改变句子的线性结构，在句子的表层结构得到体现，形成相应的句子。疑问、祈使、假设等作用于全句的功能范畴，必须在句子的三个敏感位置通过有限的句法操作手段实例化和具体化，是典型的句子功能。英汉疑问句的差异、祈使句的句法特征、祈使句的主语隐现等前人研究的热点都是句子功能在线性结构中实例化和具体化的结果，都可以推导和解释。

Sentence function is an important concept in grammar.The previous studies on sentence function focus on its pragmatic and semantic aspects，which has deviated from grammar and is not conductive to its deep research.The study on sentence function should be redirected to its syntactic aspect.Sentence function，by its definition，is a sentential functional category，forcing the independent expressive units to form a sentence，and meanwhile，as a functional feature with no phonetic forms，being realized through three sensitive positions and limited syntactic means and marking itself in language forms.This dissertation takes sentence function as the research subject，the Theory of Principles and Parameters and Language Typology as its theoretical framework，and cross-linguistic comparison as its methodology.It is held that the superficial differences among interrogatives，imperatives and hypothetical sentences result from the different instantiations of sentence function in different sensitive syntactic positions and syntactic means，a phenomenon which can be derived.There are five chapters：Chapter one is an introduction.It mainly reviews the previous studies on sentence function and points out their achievements and weaknesses.The previous studies usually give up on grammar but follow other ways.The studies of sentence fuction usually equal to the studies on the sentence moon and pragmatic classification of sentences，which lacks rigid discussion and evidence from grammatical forms.Then it introduces the theoretical framework of the dissertation—the Theory of Principles and Parameters.Finally it brings up the value of the present research and the source of the data.Chapter two is“The Nature and Function of Sentence Function”.It points out that sentence function is the research subject of sentence function.The sentence function which is realized in language forms is the sentence function in the sense of syntax.Sentence function is the peculiar function of the sentence，thus sentence function is the“difference between independent sentence and expressive clause”.In syntax，sentence function lies in［C，CP］on top of the sentence，governs the whole sentence and decides the nature of the sentence.However，sentence function and the complementizer are not the same category.They display a lot of differences and should be treated differently，thus this can solve some disputable issues as the complementizer inChina is head-initial or head-final，wh-in-situ in embedded interrogatives in English.In addition，sentence function is also different from the mood.Sentence function can be roughly described as the expressive mood.In syntax，sentence function can force clauses to become independent sentences.Chapter three，“Syntactic Means and the Scope of Sentence Function”introduces four syntactic means as“Movement”，“Marker-adding”，“Deletion”and“Reduplication”，through which the surface structure is derived from deep structure，as well as three sensitive positions as“sentence-initial”，“predicate-initial”and“sentence-final”.The criteria according to which sentence function is defined should be：1.Formally，sentence function must be realized through limited syntactic means in three sensitive positions and also must be represented in syntactic forms.2.In syntax，sentence function can force syntactic structure to become independent sentence.3.Different sentence functions must be in a substitute and complementary position.Different sentence functions cannot occur in the same sentence.Starting from these criteria，it is found that topic and focus cannot work on the whole sentence and thus they are not sentence function；tense and negation cannot force structures to become independent sentences and cannot be regarded as sentence function；Declaratives and exclamatives are not realized through limited syntactic means in three sensitive positions and thus are not sentence functions in syntax.Chapter four，“The Instantiations of the Question Feature”，points out that the question feature can force structures to become independent sentence and can be realized through limited syntactic means in three sensitive positions and thus is sentence function in syntax.The instantiations of the question feature are the different interrogatives in surface structure.The singularity of English interrogatives and the variety of Chinese interrogatives can be explained as the different parameters set in the instantiations of the question feature in languages and this phenomenon can be predicated.The research on the generative mechanisms of interrogatives can also resolve the issues as the classification of interrogatives and the nature of“KVP”question.Chapter five，“the Instantiations of the Imperative Feature and the Generative Mechanisms of Imperatives”also shows that imperative is a sentence function in syntactic sense as it can force structures to become independent sentence and can be realized through limited syntactic means in three sensitive positions.The differences in English imperatives and Chinese imperatives as there are no tense and subject in English imperatives and there may be subjects in Chinese imperatives can be derived as the superficial phenomenon as the instantiation of the imperative feature gives rise to the deletion of the tense feature and different parameter-setting in these sentence functions.Chapter six，“The Instantiation of the Hypothetical Feature and the Generative Mechanisms of Hypothetical Sentence”，takes the hypothetical feature as the sentence function in syntactic sense.The hypothetical feature works on the whole sentence，though it only brings out the existence of the embedded sentence in hypothetical sentences，it can be realized as through the limited syntactic means in three sensitive positions，and the hypothetical sentences are thus formed.The Chinese hypothetical marker“ruguo”can occur in“sentence-initial”and“predicate-initial”，but it cannot occur in“sentence-final”，while another Chinese hypothetical marker“dehua”can only occur in“sentence-final”instead of other sensitive positions.This can be explained as the different parameter-setting of sentence functions and the different instantiations of the hypothetical feature.The wh-words fail to question in hypothetical question，which by nature，is because the question feature and the hypothetical feature are both sentence functions，and thus they dispel each other and the question feature of the wh-words are restrained.Chapter seven is the summary of the present dissertation：The sentence function in its syntactic sentence is the syntactic feature which can bring out the existence of a sentence and can be realized through four syntactic means in three sensitive positions.The instantiations of sentence functions are difference sentences as we see and decide the superficial differences of the sentences.Key Words：sentence function，instantiations，syntactic means，the question feature，the imperative feature，the hypothetical feature，the generative mechanism