Why can you press "play" and "rewind" on a tape player without a tape, but it won't allow you to press record?

On any casette tape machine (audio casette, VHS tapes, etc...) there's a "record protect" tab on the back of the plastic case, opposite the tape opening. This plastic tab pushes on a lever inside the machine. If the lever's not pushed, then it prevents you from pushing the 'record' button. You can break off the little plastic tab on the tape to protect from accidently recording over it..... like the one that has your cousins wedding on it....... ;-D

The answer above is bang on, but one further addition: if you can locate the little spring in the upper left corner of the tape mechanism (push up with your finger) you'll notice that you can easily push the record button, even with no tape inside.

Two words for you my friend......Compact Disc. Forget about your tape player. You should have found the answer to that one in the 80's.