(1) Please excuse my late reply to your very friendly letter of March 1.
(2) I hope you will forgive me for not having written you for so long.
(3) I hope you will excuse me for not having replied to you until today.
(4) I humbly apologize to you for my delay in answering your kind letter of May 5.
(5) I have to (must) apologize you for not answering your letter in time.
