Ah ha, one of the things that always upsets me about OSX.
There is no "cut" for files and folders within the Finder, either Move or Copy
I guess this sets you up for some fail-safes, your data won't be gone if you were mid "Pasting" from a "cut" and the power dies...
File management within the same drive/partition:
1) The act of drag-drop alone will move the file. There is no need to use "command+drag"
2) If you want to duplicate the file. Then option+drag is what you want (as others have pointed out, a green circle with a "+" will be displayed under the curser).
File management across two different drives/partitions:
1) Drag-drop alone will duplicate the file onto the second drive by default. Option+drag not necessary.
2) If you wish to move the file (leaving no copy on the original drive), then command+drag comes into play. The green-circle with the "+" icon will disappear from beneath the cursor.
Both cases:
3) Making aliases is the same for each. alt+command+drag