‘if’ in prolog?

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Yes, there is such a control construct in ISO Prolog, called ->. You use it like this:

( condition -> then_clause ; else_clause )

Here is an example that uses a chain of else-if-clauses:

(   X < 0 ->
    writeln('X is negative.  That's weird!  Failing now.'),
    fail
;   X =:= 0 ->
    writeln('X is zero.')
;   writeln('X is positive.')
)

Note that if you omit the else-clause, the condition failing will mean that the whole if-statement will fail. Therefore, I recommend always including the else-clause (even if it is just true).

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