The expression 'if' in lambda calculus takes three arguments: x, y, and z. If x is true, it evaluates to y; otherwise, it evaluates to z. This behavior mirrors an if-then-else construct. When x is true, 'if' reduces to yz, yielding y. When x is false, it reduces to z. Thus, the 'if' expression accurately simulates an if-then-else construct by evaluating x and returning y or z accordingly