The way I always remember this is thus: If you refer to the right hand table anywhere is the where clause, you immediately turn that left join into a inner join. That's the effect you see anyway.

Putting the predicate in the ON clause keeps it a left join.

Unless of course you are doing an anti-join, where you want all records from Set A not in Set B, so you use

select cols from Set_A a

left join Set_B b

On a.col1 = b.col1

where b.col1 is NULL

That is the only time I refer to a column from the right hand table involved in a left outer join in the where clause.

HTH

Dave J