这三个都是内部语言学的分支。

phonology （语音学）是 对一种语言的语音系统的描述。主要讨论的是语音以及语音意义。（Phonology is the systematic use of sound to encode meaning in any spoken human language, or the field of linguistics studying this use）.

morphology（形态学）是对单词的结构的分析，一般认为是syntax的最小单位。morphology is the identification, analysis and description of structure of words ， and words are generally accepted as being (with clitics) the smallest units of syntax.

syntax(句法学)指的是对构造句子时应该遵循的原则。（狭义上就是语法）syntax is the study of the principles and rules for constructing sentences in natural languages。