A Japanese student once asked my American friend to write a letter of recommendation to support his application for a scholarship, saying, “I want you to write a letter of recommendation for me.” The American was shocked at the student’s rudeness. What the student probably would have said in Japanese was suisenjō wo kaite itadakitai. In English, he started with “I want…” Unfortunately for the student, he should have said, “I would appreciate it if you would….” The form “I want you to…” in English is only used by a superior to a subordinate. It is impolite in other situations, especially in the situation above.
In a similar way, when you are asking for permission to do something, such as to borrow the phone, you can ask, “Can I use your telephone?” but it is more polite to use “may”: “May I use your telephone?”