Can't tell you American British informal emphasis
Used to emphasize what you are saying
I can’t tell you how much I love you.
We can’t tell you how grateful we are for everything you’ve done for us.
I can’t tell you how gorgeous Maria is. She is too good to be true.
Used to describe the action of expressing one's opinions although others are dominating the conversation.
"Can" is used as a modal verb.
If a person has the devil's own luck, he or she has extremely good luck.
A: Yesterday I won a $10 million lottery jackpot and today I won a car in a draw.
B: You have the devil's own luck.