I’m asking about usage of “refer” and “refer to”.
Can one say “refer to the letter”?
Is “refer the letter” enough?
Refer to the letter basically means to check the letter for information. Refer the letter means send or forward the letter to someone.
Refer can mean that someone is referring to something, usually for information (e.g. refer to a letter, refer to Wikipedia, refer to the manual etc) and it can also mean to send some person to someone else for further investigation. E.g. "I will refer my patient to a heart specialist", or "I'll refer you to an hand-writing expert to analyse the letter". However, I cannot think of a valid usage case for referring an object in this way e.g. "I will refer the letter to a hand-writing expert".