Re: Let me know when she comes in.
I hadn't realized the potential ambiguity of the sentence until Ray pointed it out.
Mary has been late to work every day this week. I bet she'll be late today.
Okay - let me me know when (= what time) she comes in.
I forgot to ask Mary yesterday about whether she'll be joining us for lunch, but she'll be here any moment and I'll ask her again.
Okay - let me know (whether she will be joining us) when she comes in (because then you'll be able to ask her.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.