I split a long video into several clips with the CUT tool.
The I select a specific clip of the several cuts.
I go to the beginning, then I press "i" I then o to the end with the playhead and press "o"

1) Then when I render, it starts at some arbitrary point in the middle of the clip during playback. How is that even possible ?
2) I just cannot delete these markers . I highlight the clip wiht in and out markers, then I go to markers and delete all.
Unfortunately when I move to the end of the clip, then the out marker in blue still indicates.

Therefore delete all markers does not work at all.

How do I reset ALL markers and how do i FORCE kdenlive to render form the IN and OUT markers I selected.
kdenlive doesnt seem top have this implemented properly or I am misunderstanding something.

Markers have nothing to do with the rendering zone... are just notes placed on clips that can be moved with clips.

if you want to render a zone you need to use the SELECTED ZONE option on rendering dialog ( the zone can be expanded or moved on the timeline clicking on the small bar near the time/frame numbers)

if you want to render the entire project you have to not select "SELECTED ZONE" but "FULL PROJECT" (on render dialog)

P.S. what version of Kdenlive are you using?

Before I posted here I found countless articles that solves my problem by
Tape-head to start of clip/ press "i"
Tape-head to end of clip/ press "o"
Then render.

This seems to work for everybody on the web. So I am just wondering how they can be all wrong using it ?
This would be easy if ti worked. It worked for the first sub-clip I selected with in/out and it rendered properly only the sub-clip, however, kdenlive cannot seemingly forget the previous in & out markers and so randomly picks a wrong start/end.

How can I find out how to select a zone. Cannot figure that out.
I used this part of the manual:
https://userbase.kde.org/Kdenlive/Manual/Rendering
What is described there doesnt work either.

Tried both 20.12.0 & 19.2.X

I managed to figure out. It is the difference between the two versions. I tried to use what I was used to in 19, but 20 is slightly different.
It now has the ability to select the guides in the dropdown. After having that functionality it now works.