Can be , could be , must be ?

I think California Jim did a pretty good job, Tony.

Micawber ,
Thanks for your response.But which one is California Jim ?
The reason why I utter this issue is because I run into a sentense like "you'd better put on more clothes,or you must be catch a cold."
At the first sight ,I thought the meaning of the sentense is that "if you don't wear more clothes ,you must catch cold".
Conversely,the book told that it means "if you don't wear more clothes,it is possible for you to catch cold"
I just wondering why "must be "mean "possible" but not "certain".
I am not sure whether you guys know my points,but I hope .
Could you please help me out?I have got a bad headache about this!

Micawber ,
I have checked the other forum I post the same topic there just now,now I find who Calijim is .Thanks anyway.

No problem, Tony-- it's hard to better California Jim!