How to delete linedef only??

Sometimes when I delete a linedef it just deletes the sector the linedef is also in. 

This is before deleting the linedef by pressing Del on the keyboard

Here is after deleting the linedef with the Del key. The whole sector is just gone.

Is there a way to fix this or do we have to live with it???

Instead of deleting the line directly, select both sectors and merge them (defaultly on Shift+J I think)

52 minutes ago, Homer_hardware said:

Is there a way to fix this or do we have to live with it???

Press 'm' for 'make sector', click inside your new sector (which is technically the remnants of two sectors when you deleted the center line like that).

The reason why the sectors get destroyed is because if the sectors had different properties, there's no way to know what the "intended" properties of the merged sector would do.

If you want to make two sectors into one, use the "Merge" command on the sectors (which I do think is "Shift + J"), instead of deleting linedefs.

FWIW, I think to join sectors (merge), it's just 'j'. It's worth noting that the first sector selected in the bunch to be merged is the one from which the others will inherit their properties e.g. ceiling/floor heights, lighting, special, flats.

If only delete lines, you can use "backspace", if merge sectors do as others say. 

40 minutes ago, Jayextee said:

FWIW, I think to join sectors (merge), it's just 'j'. It's worth noting that the first sector selected in the bunch to be merged is the one from which the others will inherit their properties e.g. ceiling/floor heights, lighting, special, flats.

Join is "J," but it keeps sector boundaries intact, so it wouldn't do what the OP wanted in this case. Join is most useful for joining sectors which are physically separate.

1 hour ago, Jayextee said:

FWIW, I think to join sectors (merge), it's just 'j'. It's worth noting that the first sector selected in the bunch to be merged is the one from which the others will inherit their properties e.g. ceiling/floor heights, lighting, special, flats.

The shortcut for joining and deleting the joining lines is shift-j

1 hour ago, Misty said:

If only delete lines, you can use "backspace", if merge sectors do as others say. 

Jesus Christ, UDB has even that kind of feature?!

I believe I will never know the full power of this editor!

There's always something to impress me!

I tried it and I believe it will save me A LOT of time in future...

You can also delete vertices that way too, if needed. 

4 hours ago, Stabbey said:

 

Join is "J," but it keeps sector boundaries intact, so it wouldn't do what the OP wanted in this case. Join is most useful for joining sectors which are physically separate.

My bad, I had no idea there was the distinction -- I pretty much use 'j' (without shift) mainly for dummy/control sectors and the like.

If you made any mistakes with any DOOM level editor, you should be able to undo the action (CTRL+Z).

Merge the Sectors then delete the Vertices (DELETE) on a Linedef 1 at a time if necessary to clean up the level.