hi Dinghui, the best way I can think of to remember them is that "snug" is related to a verb "snuggle" which means to either get comfortable and cozy in like a bed or sleeping bag, or to hold another person close in an affectionate embrace -- so snug is the way you feel when you are snuggled :-) You could associate "smug" with "mug" which is usually a cup, but is also a term for someone's face, like a mug shot. We commonly say someone has a smug look on their face, related to what Jose said, they are too sure of themselves.