I cannot help telling him the truth.

[angliholic]

I cannot help telling him the truth.:up:
I cannot help but tell him the truth.:up:
I cannot help tell him the truth.:?:
I cannot help but telling him the truth.:?:



Could you come up with a reason why the last two don't sound right? Thanks.

 

[susiedqq]

I cannot help telling him the truth.:up:
I cannot help but tell him the truth.:up:

means:

I cannot help (except by) telling him the truth.:?:

I cannot help (in any other way) but (instead I can) tell him the truth.:?: