函的用法和样例：
例句
-
忽视银行经理来函，後果堪虑。
One ignores letters from the bank manager at one's peril. -
该函已退给（我方）要求对某问题加以解释。
The letter was referred back (to us) with a query. -
我们收到了您15日来函。
We are in receipt of your letter of the 15th. -
她今天早晨收到了几封公函。
She received several official letters this morning. -
商业信函必须是正式的，但是写信给朋友应写得自然一些。
Business letters must always be formal, but we should write in a natural way to friends.
