SQL tuning questions

hi, guys:

   Q 1: why some time oracle won't use the hint which I give?    Q 2: since I found the "set timing on" cann't tell the exact the time the oracle

            execute the query, so how can I tell the exact time of execution of the query?

    Q3: I have no problem with each single simple tuning sample SQL, but in real life,

             the SQL became very complicate and huge, so is there any steps or spots that I

             must look or deal first, and then second....... ? please give some suggestion of

             the real life SQL tuning. thanks

                                             tshen