Two rectangles of different sizes should be aligned horizontally on their left edge.
Theoretically, I would select the two rectangles and then click on an "Align"-button. This is how I know it from other graphics programs.
I can't find a way to do this in the menus of LibreCad.
I hope to have found the right subforum for my question.
Look at the status bar (bottom) or the command line: you are asked to define reference points. Press Align first, then select the entities, close selection with Enter. The command probably doesn't work exactly the way you expect but does it's purpose.