> Senior members from suggest me that before 9i the optimizer was rule based
No, the CBO was introduced in Oracle 7.
> so can you please tell me that if we wants to use hints then when to use them>
Hints can be handy during development for tuning and tweaking (but not twerking).
In production code, hints can be poison. Some of the worst performance problems I've seen were *caused* by inappropriate hints.
Keep in mind I'm talking about "tuning hints". There are some other hints (like insert /*+ append */) that fall into a totally different category.
Hints are a path of last resort.
I suggest you do some searches on http://asktom.oracle.com and you'll get tons of examples and explanations regarding performance tuning, as well as hints.
Also, you should read the Oracle Performance Tuning Guide, it comes in 2 flavors:
2-day Guide: Contents
Full Guide: Contents