请问什么时候用for如:buy sth for sb 什么时候用to如：show sth to sb 力求要细,

请问什么时候用for如:buy sth for sb 什么时候用to如：show sth to sb 力求要细, give sth to sb ,give sth for sb请问什么时侯,用to什么时候用for.请说的明白一点, 英语短语记法有些短语里面 比如 buy sth for sb 有些短语sb前面是加的for 什么时候 sb之前用to 什 buy sth for sb 还是 buy sth to sb give sth to sb buy sth for sb buy sth.to sb.=buy sb.sth还是 buy sth.for sb.=buy sb.sth 请问是buy sth to sb还是buy sth for sb? bring sth for/to sb等于bring sb sth buy sb sth等于buy sth for sb 可以说buy sth for sb,get sth for sb,give sth to sb吗 help sb (to) do sth 什么时候要用to? sth to sb 还是 sth for sb sth to sb 和sth for sb