Perfect tenses relate two areas of time in some way, where simple tenses are primarily isolated in a single time zone:

I haven't seen Saddam. (I did not see him before, though I still have a possibility of seeing him -- this idea connects the past with the present and future.)

I didn't see Saddam. (At some undefined but specific instance(s) isolated in the past, I missed him. This statement makes no reference to future possibility in other time zones.)



Would /should / could can appear as past forms of will / shall / can, but are more often used in ameliorative ways, to express emotional distance, euphemism, diffidence, courtesy, condition.

I can hear you sing in the shower. (present-- or actually, timeless-- it refers to any time in the present or future)

I could hear you sing in the shower. (past)

I could scrub your back if you would like me to. ('could' is timeless; it offers a possibility not a fact, and is more polite than 'can'.)



If you will search our threads, Dono, you will find many discussions on modals.