What the difference between bildim and biliyordum

why sometimes (iyor+di)

but sometimes (id) only

and I know (iyor) is present simple form

thanks in advance

-Iyor shows that the action is continuous or progressive. It is not a present tense marker because present tense is unmarked.

Combined with the past tense marker -DI- it denotes a past tense action which was continuous or gradual. Like Kitap okuyordum ´I was reading a book/books, I took my time, it doesn´t matter if I completed it or not because it is not the issue here´.

Thanks very much Abla

Hi Nifrtity, your heading says "Past perfect", but the tense you are writing about is called "past continuous" in English.

Click here for a site that explains this tense in English.

The past tense (di) is used when you are talking about something that has happened.

If something was happening continuously for a period, but was interrupted or has now stopped, you can use the past continuous tense (iyordu)

Araba çok hızlı gidiyordu, birden lastik patladı.

The car was going very fast, and suddenly the tyre burst. 

Another example

You were doing something continuously in the past (watching televizion), and something happened (your mother rang) which caused you to stop the continuous action.

I was watching televizion when my mother called. 
Annem aradığında televizyon izliyordum .

The English past perfect tense (had .....ed) is often expressed by adding mişti in Turkish, but this is another subject that is not simple.

Thanks so much

It´s my pleasure to help another learner