Ending a Motion Tween; Having Trouble

I’ve always been plagued with this problem: How do I completely end a motion tween. Sometime when I start a new key-frame right beside I previous motion tween, and try to drag a new symbol into it, it swaps it with the symbol from my previous motion tween. It is very frustrating and I’ve tried to find a way to solve it, but I didn’t know what to google.

All that I know about this problem is that the motion tween timeline looks different when it’s doing what I described above. It doesn’t put a black vertical line before the black dot (that signifies a new key-frame). I’ll draw it to give you an idea:

The bad way:
o>--------->o--------->o|

The way I want it:
o>-------->|oo-------->|o
…^This being the new symbol.

I sure hope this is something simple that I just failed to learn while learning Flash on my own. Thanks for the help, all.