how can I clone an object in Illustrator so that a link remains to the source object and modifying it alters all the copies as well? Thanks
A couple of ways.....
Method 1: For live paths you want to alter
For example, working on a symmetrical image.
Select the object.
Choose Effect > Distort & Transform > Transform
Enter a number greater than 0 in the "copies" field.
Then tick a reflect box or adjust some other transform option in the dialog window.
Method 2: For almost any object.
Select the object and click the "New" icon on the bottom of the Symbols Panel.
When you need a copy.... click and drag the symbol from the Symbols Panel to your artboard.
When you want to edit the artwork, double click one of the symbols on the artboard or the symbol in the Symbols Panel. You will enter Symbol Editing Mode allowing you to alter the original artwork and any changes made will be reflected in all symbols when you exit the editing mode.
You can also individually transform or distort the symbols without altering the original symbol artwork. There are also various symbol editing tools in the toolbar such as the Symbol Stainer, Symbol Shifter, etc.