cuando se coloca 's
cuando de coloca apostrofe s
Principalmente: a) si se omite las letras (normalmente para unir dos palabras); o b) para demostrar que un objecto pertenece a alguien/algo.
Mainly a) if you omit some letters (usually by joining two words); or b) to show that an object belongs to something/someone.
Por ejemplo/for example:
1) That is my pen = that's my pen
They are really happy = they're really happy
2) This pen belongs to my friend = this is my friend's pen
That doctor has a red coat = that doctor's coat is red
Sorry if this is too simple, I can't tell how good your English is from your profile