How to use the ELSIF statement in Postgres

I'm wondering if I can run the if statement by itself, or it can't stand on its own, has to be in a nested statement. I can't directly run the following code.

IF tax_year=2005 THEN
UPDATE table1 SET column1=column1*3;
ELSIF tax_year=2006 THEN
UPDATE table1 SET column1=column1*5;
ELSIF tax_year=2007 THEN
UPDATE table1 SET column1=column1*7;
END IF;

Also, I didn't write it out that when tax_year=2008, column1=column1. I'm not sure if it needs to be in the code since column1 won't change in 2008.

Thanks for your help!