I have to apologize, she answered her letter with a slow. In fact, I have read a letter from the date I received, but I did not have time to respond to her letter back, because this range I learned very hard.
I have to apologize to her by her late replying. In fact, I've read since I've already been. But I have no time to answer her letter back because this period I studied very hard.
I owe you an apology that answer your letter slowly. In fact I had read the letter, since the day I got it. But I have no time to answer her letter back because I've been studying very hard.