5.I'm sorry I'm replying so late.
很抱歉，给您回复晚了。

I'm sorry I didn't write you back sooner.
很抱歉，没有及早给您回信。
I apologize for the lateness of this reply.
未能及时回复您，我感到非常抱歉。

6.I meant to reply sooner.
我本打算尽快回复您。

I meant to reply sooner but things have been pretty hectic lately.
我本打算尽快回复您，但最近事物太多了。
I've been meaning to reply to your e-mail but I never got around to it.
我一直想给您回复邮件，但一直抽不出时间。