為什麼要用for而唔用to???(請解釋)

Q1.Dad bought a toy for me.為什麼要用for而唔用to???(請解釋)
For using "for", it means that Dad bought a toy because of you.
For using "to", it means Dad bought the toy and gave you.
Both of them can be used, unless you have fixed meaning.
Q2.Can you be my friend?為什麼要用be而唔用do???(請解釋)
For using "be", it's commonly used beacuse it means whether the one can be your friends or not.
For using "do", it's used in very small examples, unless the menaings is fixed, otherwise, I will not recommand you to use "do"

Q1 用TO唔係唔得(意思係給你 )，但係表達唔到DAD係為你 買玩具。通常為你/人好都會用FOR。

Q2 用DO係廣東話直譯英文，所以都係死記用BE吧。