can't teach an old dog new tricks的意思是老狗变不出新把戏、老狗学不会新把戏。

【例句】
My father will never change：you can't teach an old dog new tricks.
我爸爸永远不会改变，真是老狗学不会新花样。

Unfortunately, there is some truth to the old statement "you can't teach an old dog new tricks." unless a person is used to changing, it's hard to change.
遗憾的是有一些古老的说法仍然颇有道理，像老狗学不会新把戏，除非一个人习惯于变化，否则改变自我是十分困难的。