Huhh takut nih takut salah englishnya... maaf ya

Hhmm guys, can you explain to me what's the different "un" and "im" and when exactly we should use that words? I am sorry for my terrible english

Example "unpolite or impolite" ??

Sorry yang ini gatau nulis englishnya gimana
Kenapa kata possible lawannya IMpossible??
Kenapa kata happy lawannya UNhappy?

Thank you agan aganwati