How to join two tables with different numbers of rows
When joining you join one key to another x=y but you seem to want to join x=y-1 instead. In your case you want to join 1 with 0 and 2 with 1. This is not a good practice. When joining you always you should join keys with eachother without chaning them.

You should think about what you really want as a result because it is not a logical join to do. But to genereate the result you want you can do like this:  

select a.*, b.* from t1 a LEFT OUTER JOIN t2 b on a.rnum-1 = b.rnum


DB Fiddle

This is a good reference image for future use: