If

gives t if condition evaluates to True, and f if it evaluates to False.

gives u if condition evaluates to neither True nor False.

Out[2]=

If the condition is neither True nor False, If remains unevaluated:

The form with an explicit case for an undetermined condition evaluates in any case:

Use TrueQ to force the condition to always return a Boolean value:

Define a piecewise function with both If and Round as piecewise elements:

Use PiecewiseExpand to get it into piecewise normal form:

Perform several different symbolic operations:

If can be used as a statement:

It can also be used as an expression returning a value:

Only the branch actually taken is evaluated:

Define a function by cases:

Alternatively, use several conditional definitions:

Use Which rather than a nested if-then-elsif chain: