[oracle] If statement within Where clause

I am working with a query which contains "IF" statements within a "WHERE" clause. But PL\SQL Developer is giving some errors while executing it. Can anyone please help me with the correct query? Here is the query:

SELECT t.first_name,
       t.last_name,
       t.employid,
       t.status
  FROM employeetable t
 WHERE IF status_flag = STATUS_ACTIVE then t.status = 'A'
       IF status_flag = STATUS_INACTIVE then t.status = 'T'
       IF source_flag = SOURCE_FUNCTION then t.business_unit = 'production'
       IF source_flag = SOURCE_USER then t.business_unit = 'users'
   AND t.first_name LIKE firstname
   AND t.last_name LIKE lastname
   AND t.employid LIKE employeeid;

I receive the error "ORA-00920: invalid relational operator".

Placing brackets around status_flag = STATUS_ACTIVE results in error "ORA-00907: missing right parenthesis"

You can't use IF like that. You can do what you want with AND and OR:

SELECT t.first_name,
       t.last_name,
       t.employid,
       t.status
  FROM employeetable t
 WHERE ((status_flag = STATUS_ACTIVE   AND t.status = 'A')
     OR (status_flag = STATUS_INACTIVE AND t.status = 'T')
     OR (source_flag = SOURCE_FUNCTION AND t.business_unit = 'production')
     OR (source_flag = SOURCE_USER     AND t.business_unit = 'users'))
   AND t.first_name LIKE firstname
   AND t.last_name  LIKE lastname
   AND t.employid   LIKE employeeid;