I have to ask this. What is the difference between -miş and -mişti. I mean,the both are past,right?

-mIş  -  past (+ hearsay), often equals to English past tense

-mIştI  -  double past, often equals to English pluperfect

Sağol. And what is the difference between English past tense and English pluperfect? For pluperfect I first heard now in my life. :O

Sorry. They seem to use the name past perfect these days. Look here:

                           http://www.learnenglish.de/grammar/tensetext.htm

Sorry? These days?

I was taught it is pluperfect. That was some years after WW1.

There are some lovely schemas on that page. I wish I could find them for Turkish. =)

Oh,that. Okay Thank you. I´ll watch more Muhtesem,and I´ll read this. I hope it would be helpful.
For example,i heard them to say:
Ben size söz vermiştim. - I promised you. OR
Söylemiştim. - I said.
As I know,they use -miş for something that they not sure that´s really happened. But if he/she knows that he/she promis something,I don´t get why they have to use -miş(ti)... 

Then you must read this thread, nemanja:

                http://www.turkishclass.com/forumTitle_53653

tunci´s posts are especially about your problem.

Tunci´s posts are always helpful. I guess it will be too this time. Inşallah.

I call that form as Narrative perfective tense. In Turkish -mIştı  marking is used for actions that are completed in the past. By using - mIştı , we are ;

1. narrating the past action to someone

2. I witnessed or I myself  performed that action.

3. The action occurred in the definite past *

As in your example; 

Ben size söz vermiştim. ---> I had promised you.

1. I am reminding you, narrating you the event [my promising you].

2. I remember and i witnessed the action that it has been performed by me.

3. It occurred in the past and it occurred in a definite time in the past.

==============================================

The only distinctive difference between simple past tense [size söz verdim] and [size söz vermiştim] is ,  we are narrating the past event that we witnessed.

==============================================

3. 3. The action occurred in the definite past *

Although this form technically should indicate the indefinite past because of tense narrowing at present time it is used for definite past.

 According to Prof.Hamza Zülfikar ;

The definite perfective past    -dıydı    is replaced by  indefinite perfective past - mıştı 

söz verdiydim ---------------------------------> söz vermiştim

                           tense narrowed into

- as a result , at present time, we use -mişti form for definite perfective past time.

The nearest tense in English is  plu-perfect tense [had]