Oracle SQL developer error on IF condition in where caluse

I got error in sql developer query as below.Please let me know what is reason for this.

select * from homedeliveryorderheader where if (:st =0 then lastmodifiedby is not null) end

ORA-00907: missing right parenthesis 00907. 00000 - "missing right parenthesis" *Cause:
*Action: Error at Line: 19 Column: 60