这些概念的事实基础是:

表达式:一种语法类别，其实例可以求值。

语句:一种语法类别，其实例可能涉及表达式的求值，并且不能保证求值的结果值(如果有的话)可用。

除了最初几十年的FORTRAN上下文之外，公认答案中表达式和语句的定义显然都是错误的:

Expressions can be unvaluated operands. Values are never produced from them. Subexpressions in non-strict evaluations can be definitely unevaluated. Most C-like languages have the so-called short-circuit evaluation rules to conditionally skip some subexpression evaluations not change the final result in spite of the side effects. C and some C-like languages have the notion of unevaluated operand which may be even normatively defined in the language specification. Such constructs are used to avoid the evaluations definitely, so the remained context information (e.g. types or alignment requirements) can be statically distinguished without changing the behavior after the program translation. For example, an expression used as the operand of the sizeof operator is never evaluated. Statements have nothing to do with line constructs. They can do something more than expressions, depending on the language specifications. Modern Fortran, as the direct descendant of the old FORTRAN, has concepts of executable statements and nonexecutable statements. Similarly, C++ defines declarations as the top-level subcategory of a translation unit. A declaration in C++ is a statement. (This is not true in C.) There are also expression-statements like Fortran's executable statements. To the interest of the comparison with expressions, only the "executable" statements matter. But you can't ignore the fact that statements are already generalized to be constructs forming the translation units in such imperative languages. So, as you can see, the definitions of the category vary a lot. The (probably) only remained common property preserved among these languages is that statements are expected to be interpreted in the lexical order (for most users, left-to-right and top-to-bottom).

(BTW，关于C的材料，我想补充一下[引文]，因为我不记得DMR是否有这样的意见。似乎不是，否则就没有理由在C语言的设计中保留功能重复:特别是逗号操作符和语句。)

(以下基本原理并不是对最初问题的直接回应，但我觉得有必要澄清这里已经回答过的一些问题。)

然而，在通用编程语言中，我们是否需要特定类别的“语句”是值得怀疑的:

Statements are not guaranteed to have more semantic capabilities over expressions in usual designs. Many languages have already successfully abandon the notion of statements to get clean, neat and consistent overall designs. In such languages, expressions can do everything old-style statements can do: just drop the unused results when the expressions are evaluated, either by leaving the results explicitly unspecified (e.g. in RnRS Scheme), or having a special value (as a value of a unit type) not producible from normal expression evaluations. The lexical order rules of evaluation of expressions can be replaced by explicit sequence control operator (e.g. begin in Scheme) or syntactic sugar of monadic structures. The lexical order rules of other kinds of "statements" can be derived as syntactic extensions (using hygienic macros, for example) to get the similar syntactic functionality. (And it can actually do more.) On the contrary, statements cannot have such conventional rules, because they don't compose on evaluation: there is just no such common notion of "substatement evaluation". (Even if any, I doubt there can be something much more than copy and paste from existed rules of evaluation of expressions.) Typically, languages preserving statements will also have expressions to express computations, and there is a top-level subcategory of the statements preserved to expression evaluations for that subcategory. For example, C++ has the so-called expression-statement as the subcategory, and uses the discarded-value expression evaluation rules to specify the general cases of full-expression evaluations in such context. Some languages like C# chooses to refine the contexts to simplify the use cases, but it bloats the specification more. For users of programming languages, the significance of statements may confuse them further. The separation of rules of expressions and statements in the languages requires more effort to learn a language. The naive lexical order interpretation hides the more important notion: expression evaluation. (This is probably most problematic over all.) Even the evaluations of full expressions in statements are constraint with the lexical order, subexpressions are not (necessarily). Users should ultimately learn this besides any rules coupled to the statements. (Consider how to make a newbie get the point that ++i + ++i is meaningless in C.) Some languages like Java and C# further constraints the order of evaluations of subexpressions to be permissive of ignorance of evaluation rules. It can be even more problematic. This seems overspecified to users who have already learned the idea of expression evaluation. It also encourages the user community to follow the blurred mental model of the language design. It bloats the language specification even more. It is harmful to optimization by missing the expressiveness of nondeterminism on evaluations, before more complicated primitives are introduced. A few languages like C++ (particularly, C++17) specify more subtle contexts of evaluation rules, as a compromise of the problems above. It bloats the language specification a lot. This goes totally against to simplicity to average users...

为什么是语句?不管怎样，历史已经一团糟了。似乎大多数语言设计者都没有仔细选择。

更糟糕的是，它甚至让一些类型系统爱好者(他们对PL历史不够熟悉)产生了一些误解，认为类型系统必须与操作语义上更基本的规则设计有重要关系。

严肃地说，基于类型的推理在许多情况下并不是那么糟糕，但在这个特殊情况下尤其没有建设性。即使是专家也会把事情搞砸。

For example, someone emphasizes the well-typing nature as the central argument against the traditional treatment of undelimited continuations. Although the conclusion is somewhat reasonable and the insights about composed functions are OK (but still far too naive to the essense), this argument is not sound because it totally ignores the "side channel" approach in practice like _Noreturn any_of_returnable_types (in C11) to encode Falsum. And strictly speaking, an abstract machine with unpredictable state is not identical to "a crashed computer".

表达式可以求值得到一个值，而语句不返回值(它们是void类型)。

当然，函数调用表达式也可以被视为语句，但除非执行环境有一个特殊的内置变量来保存返回值，否则无法检索它。

面向语句的语言要求所有过程都是语句列表。面向表达式的语言，可能是所有的函数式语言，都是表达式的列表，或者在LISP的情况下，是一个表示表达式列表的长s表达式。

尽管这两种类型都可以组合，但只要类型匹配，大多数表达式都可以任意组合。每种类型的语句都有自己的方式来组合其他语句，如果它们可以做到这一切的话。Foreach和if语句要么要求单个语句，要么要求所有子语句一个接一个地放入语句块中，除非子语句允许它们自己的子语句。

语句还可以包括表达式，而表达式实际上不包括任何语句。不过，lambda表达式是一个例外，它表示一个函数，因此可以包括函数可以包含的任何东西，除非语言只允许有限的lambdas，比如Python的单表达式lambdas。

在基于表达式的语言中，你所需要的只是一个函数的单个表达式，因为所有的控制结构都返回一个值(其中很多返回NIL)。不需要return语句，因为函数中最后求值的表达式就是返回值。

很简单:表达式的值是一个值，而语句不是。

最准确地说，语句必须有“副作用”(即命令式)，表达式必须有值类型(即不是底部类型)。

语句的类型是单元类型，但由于停止定理，单元是虚构的，所以我们说底层类型。

Void并不是最下面的类型(它不是所有可能类型的子类型)。它存在于没有完整的声音类型系统的语言中。这听起来可能有点势利，但是完整性(如方差注释)对于编写可扩展软件是至关重要的。

让我们看看维基百科对这件事是怎么说的。

https://en.wikipedia.org/wiki/Statement_ (computer_science)

在计算机编程中，语句是命令式编程语言中最小的独立元素，它表示要执行的某些操作。 许多语言(例如C语言)区分语句和定义，语句只包含可执行代码和声明标识符的定义，而表达式只计算值。

表达式是产生一个值的任何东西:2 + 2 语句是程序执行的基本“块”之一。

注意，在C语言中，“=”实际上是一个运算符，它做两件事:

返回右边子表达式的值。 将右边子表达式的值复制到左边的变量中。

下面是一段ANSI C语法的摘录。你可以看到C语言没有很多不同种类的语句……程序中的大多数语句都是表达式语句，即结尾带有分号的表达式。

statement
    : labeled_statement
    | compound_statement
    | expression_statement
    | selection_statement
    | iteration_statement
    | jump_statement
    ;

expression_statement
    : ';'
    | expression ';'
    ;

http://www.lysator.liu.se/c/ANSI-C-grammar-y.html

语句是表达式的一种特殊情况，具有void类型。语言区别对待语句的倾向经常会导致问题，如果恰当地概括它们会更好。

例如，在c#中，我们有非常有用的Func<T1, T2, T3, TResult>重载泛型委托集。但是我们也必须有相应的Action<T1, T2, T3>集合，并且通用的高阶编程必须不断重复来处理这种不幸的分叉。

简单的例子——在调用另一个函数之前检查引用是否为空的函数:

TResult IfNotNull<TValue, TResult>(TValue value, Func<TValue, TResult> func)
                  where TValue : class
{
    return (value == null) ? default(TValue) : func(value);
}

编译器能处理TResult为空的可能性吗?是的。它所要做的就是要求return后面跟着一个void类型的表达式。default(void)的结果将是void类型，传递的func将需要是func <TValue, void>(这将等效于Action<TValue>)的形式。

其他一些答案暗示你不能像连接表达式那样连接语句，但我不确定这个想法是从哪里来的。我们可以想到;它作为二进制中缀运算符出现在语句之后，取void类型的两个表达式并将它们组合成一个void类型的表达式。