Can you use IF in SQL query

You can use if statement  to test for a condition

From BOL:

IF (@ErrorSaveVariable <> 0)
BEGIN
   PRINT 'Errors encountered, rolling back.'
   PRINT 'Last error encountered: ' +
      CAST(@ErrorSaveVariable AS VARCHAR(10))
   ROLLBACK
END
ELSE
BEGIN
   PRINT 'No Errors encountered, committing.'
   COMMIT
END
RETURN @ErrorSaveVariable

Case statement evaluates a list of conditions and returns one of multiple possible result expressions.

Let me clarify,

I just want to run a simple SELECT query and I want to know if I can use IF or if I can only use CASE

For example, this works fine:

SELECT
     CASE WHEN [Sales] IS NULL THEN 0 ELSE [Sales] END as Expr1
FROM TABLE1

If I try
SELECT
     IF [Sales] IS NULL
          THEN 0
          ELSE [Sales]
     END IF AS Expr1
FROM TABLE1

I get an error....

In this instance, you can't use IF.   You want to use ISNULL instead:

Select ISNULL(Sales, 0)
from Table1