I'm having a problem on Solidworks 2018, I made an assembly (a subassembly) that "works", i.e. by making it physically move with the mouse, so it's a mechanical system made. Afterwards, I imported this subassembly into another assembly (which therefore constitutes the entire mechanical system). Once this import is done, I realize that I can't physically move this subassembly in the general assembly! I don't know why? Is there a control that prevents the subassembly from moving? I don't understand.
Thank you for your answers, and if necessary I can give you more details.
Does your subassembly have "(f)" in front of its name in the tree? If necessary, right-clicking on the name and then "Free" will change this "(f)" to "(-)" and should allow you to move your subassembly freely.
rpicard's answer is the right one. You have to make a subassembly flexible in an assembly so that it can be free to move and for that you just have to right click on it in the creation tree and click on Flexible and it will be free as in its assembly.
A little thing to know when using the Flexible function is that at times the assembly becomes rigid again without you really knowing why (in general it's due to constraints that it loses, it doesn't like certain types of constraints at all). In this case, simply make it rigid again and then put it back on flexible immediately. In general, everything goes back in order and you can move your assembly again.