What is the difference between

                    Uzun zamandır seni görmüyorum

                    AND Uzun zamandır seni görmedim ?

Is it possible to say

                    ?Uzun zamandır seni görmüyordum ?

 Hmmm for me ;

1- Uzun zamandır onu görmüyordum --> I saw the person whom i had not been seeing for a long time.  (Onu uzun zamandır görmüyordum would be a proper sentence)

2-  Uzun zamandır onu görmüyorum --> I still dont see him. I have not been seeing him for a long time.   ( Onu uzun zamandır görmüyorum)

3-  Uzun zamandır onu görmedim ?  -->  I still dont see him. I have not been seeing him for a long time. (Onu uzun zamandır görmedim)

First sentence is clear though. For 2 and 3, i dont see a big difference in the meaning. But i would prefer using 2nd sentence instead of 3rd.

Because if we make the sentences positive;

Onu uzun zamandır görüyorum. ( I have been seeing him for a long time)

Onu uzun zamandır gördüm.  ( No meaning) (But it still doesnt mean the negative form of the sentence is incorrect.)

Maybe there can be others who can see i dont see. We will wait and see.

Interesting, Umut, and it makes sense also. I have understood -DI past is the tense with the highest profile in Turkish. It is both

                             a) absolute past AND

                             b) perfective (completed) aspect.

I think the starting point is here:

The expression uzun zamandır matches with progressive aspect. If you try to bring -DI- past here it is meaningless. But strangely enough it works with negation and I wonder why:

Just like it has to do with the action sphere of the negation but I cannot catch the thought now.

Anyway, I wanted to know the differences and now I do. Thank you.

Uzun zamandır seni görmüyordum 

1.) Think that you have just seen a person who you haven´t seen for a long time. Or maybe you have just heard his voice, so you made this sentence;

Görmüyordum ama şimdi gördüm

2.) think that you have just realized this; (the event/reality that I haven´t seen you) şimdi aklıma geldi (I have just realized it!)

When you put the past -dı suffix to this, I think we understand either;

1.) that the action has just finished.

2.) that you jave just realized that it happens or maybe it happened and finished.

Yes, that is what aspectual elements basically bring to the sentence: the speaker´s standpoint to the state of affairs. Thanks for pointing it out, caliptrix.

Tense and aspect go hand in hand in Turkish. That is why they are difficult.

I can not really think of an appreciable difference between the first two sentences. The third sentencence may convey a very similar meaning, but there is a circumstential difference. between the fist two and the third sentences.

The speaker can use the first two sentences to some one he/she has not seen for a long time, but is now talking on the phone. There is still no physical "seeing", but a one-to-one contact has been made.

The third sentence would be used when the two have finally met and actually see each other.