The basic conditionals:

if ( ..a ? true: ( ..a -- ..b ) false: ( ..a -- ..b ) -- ..b )

when ( ..a ? true: ( ..a -- ..a ) -- ..a )

unless ( ..a ? false: ( ..a -- ..a ) -- ..a )

Forms abstracting a common stack shuffle pattern:

if* ( ..a ? true: ( ..a ? -- ..b ) false: ( ..a -- ..b ) -- ..b )

when* ( ..a ? true: ( ..a ? -- ..a ) -- ..a )

unless* ( ..a ? false: ( ..a -- ..a x ) -- ..a x )

Another form abstracting a common stack shuffle pattern:

?if ( ..a default cond: ( default -- new/f ) true: ( ..a new -- ..b ) false: ( ..a default -- ..b ) -- ..b )

?when ( ..a default cond: ( ..a default -- ..a new/f ) true: ( ..a new -- ..a x ) -- ..a default/x )

?unless ( ..a default cond: ( ..a default -- ..a new/f ) false: ( ..a default -- ..a x ) -- ..a default/x )

Sometimes instead of branching, you just need to pick one of two values:

? ( ? true false -- true/false )

Two combinators which abstract out nested chains of if:

cond ( assoc -- )

case ( obj assoc -- )

Expressing conditionals with boolean logic

See also
Booleans, Bitwise arithmetic, both?, either?