Things that need impovement in my opinion:
1. When you activate the Modular Building System, it automatically attaches e.g. a plank on top of another one. However, when you now deactivate the system to be able to rotate the plank, it will happen that the planks don't touch each other at their ends anymore.
2. When you press the right [Ctrl], you are able to place an item wherever you want, but when you disable the system again, it will reset the position.
This leads to at least two problems:
a) You have to rotate an item correctly before you can manually place it. This is because you can not rotate items while the Manual positioning system (right [Ctrl]) is active. If you want to change the rotation, you have to disale this system before which will reset the manual position of the item.
b) Let's say you want to place a row of planks. You also want to set the first plank very precisely with the right [Ctrl] - system. You can do that, but you have to hold mouse one already before you press [Ctrl] again, otherwise it will reset the first plank and you can not place the rest of the planks neither. This is because you can not place rows while using the manual positioning.
My suggestions:
1. Change the Modular Building the way, that an item will still be attached to the end of another one, if you turn it off and you don't move. So if you turn it off and use the arrrow keys to rotate it, the two planks will still be attached to each other as much as possible, even though e.g. the top plank in this picture will be tilted towards the player. Once the player moves (with w,a,s and d), the attachement will automatically be turned off (so that you can place the object elsewhere).
2. I would recommend to divide the two funtions the right [Ctrl] button currently has to actually two keys:
The first one should simply make the item freeze, so that the player can walk around it without that it moves at all, however nothing else should happen. This could still be bind to the [Ctrl] button. This system should work no matter if either the Modular Building or the Manual Positioning are active or not.
The second button should toogle the Manual Positioning system on or off. Or more precisely: It should toogle between the 'standard' mode which allows you to resize and rotate items with the shift and arrow keys, and the Manual Positioning system. One of these modes should always be on, when you equip a Construction Element, since the first one I mentioned is always on at the moment, as well. Obviously, this idea would require another key. I would suggest useing the right shift key for this and only keep the left one for resizing items. Of course it would be the best, if everyone could manually set a key for this 
If red would implement all these suggestions, this would mean for all players:
New:
- You could move items with the Manual Positioning system, but when you move (with w,a,s and d), the item would move as well.
- You could rotate an item whenever you want, without that its position will change.
- You would be able to freeze an item and move around it without it moving as well, but you could still both rotate/ size and position it.
Old, but still working then:
- You could move items with the Manual Positioning system, but when you move (with w,a,s and d), the item would not move as well
This suggestion would require another thing as well, that I have requested before, because otherwise this would lead to more problems:
You should clearly see, when the Modular Building, the Manual Positining and the new 'extracted' 'Freeze mode' (= an item does not move when the player moves) are active. Otherwise, it could be quiet frustrating, when you always have to test, what system is actually active.
Specificly, I would suggest:
- An icon or note on should appear in the corner of the screen when one of these systems is active.
- When the Modular Building system is active and you are trying to attach an object to another, the end where you try to attach something should be colored, e.g. like shown in this picture.
- Maybe the controls and the functions of the arrow keys and the shift key should appear on the screen (and not just in the [F1] help section), to a) give feedback that these systems are active and b) so that you can not forget the controls. This is because even I who have used these systems for quiet a while, sometimes do things wrong The notes should obviously be quiet small, so that they don't take so much screen. But I think that when you are building, the focus is more on placing objects precisely, rather than having a great view on everything. Maybe the help window should then show some more detailed instructions instead.
- Maybe when you 'freeze' an item (= it does not move when you do), the edges of it should blink in a 'freezing' blue, so that you can see, that it is >frozen< xD
