Add definitions for expressions and statements to C# fundamentals program structure (#46896)

* Initial plan for issue

* Add definitions for expressions and statements to C# fundamentals program structure

Co-authored-by: BillWagner <[email protected]>

* Improve flow of expressions and statements section per review feedback

Co-authored-by: BillWagner <[email protected]>

* Apply style guide feedback: use italics for expressions/statements and improve clarity

Co-authored-by: gewarren <[email protected]>

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BillWagner <[email protected]>
Co-authored-by: gewarren <[email protected]>

Author: 3 people

docs/csharp/fundamentals/program-structure/index.md

@@ -19,6 +19,29 @@ You can also create a static method named [`Main`](main-command-line.md) as the
 
 In that case the program will start in the first line of `Main` method, which is `Console.WriteLine("Hello world!");`
 
+## Expressions and statements
+
+C# programs are built using *expressions* and *statements*. Expressions produce a value, and statements perform an action:
+
+An *expression* is a combination of values, variables, operators, and method calls that evaluates to a single value. Expressions produce a result and can be used wherever a value is expected. The following examples are expressions:
+
+- `42` (literal value)
+- `x + y` (arithmetic operation)
+- `Math.Max(a, b)` (method call)
+- `condition ? trueValue : falseValue` (conditional expression)
+- `new Person("John")` (object creation)
+
+A *statement* is a complete instruction that performs an action. Statements don't return values; instead, they control program flow, declare variables, or perform operations. The following examples are statements:
+
+- `int x = 42;` (declaration statement)
+- `Console.WriteLine("Hello");` (expression statement - wraps a method call expression)
+- `if (condition) { /* code */ }` (conditional statement)
+- `return result;` (return statement)
+
+The key distinction: expressions evaluate to values, while statements perform actions. Some constructs, like method calls, can be both. For example, `Math.Max(a, b)` is an expression when used in `int result = Math.Max(a, b);`, but becomes an expression statement when written alone as `Math.Max(a, b);`.
+
+For detailed information about statements, see [Statements](../../programming-guide/statements-expressions-operators/statements.md). For information about expression-bodied members and other expression features, see [Expression-bodied members](../../programming-guide/statements-expressions-operators/expression-bodied-members.md).
+
 ## Related Sections
 
 You learn about these program elements in the [types](../types/index.md) section of the fundamentals guide: