There are three ways to copy an object. First select the object by double-clicking on it:
Right click the cube. Select the Copy tool in the Context Menu and then right click anywhere else in the scene and choose the Paste button. (This also works with CTRL + C or V). Use Paste In Place (CTRL+Shift+V) to paste an object exactly where it was copied from.
Hold the CTRL key down and start dragging the object - a copy will be available to drag.
Right click the cube and select the Array tool. You can select the number of copies and whether the next drag will define the distance between two objects or the total length of the array.