自作聪明英语

自作聪明这个词语用英语表达翻译为：fancy [think] oneself clever [smart]

One should know how much he knows, he who pretends to know everything will not only do harm to others but himself

One should know his own capability, a wiseacre man will finally hurt not only others but also himself.

don't smart yourself.

People should konw their limitations / konw themselves very well. Ultimately those smarty-pants will harm others and themselves.