Thanks,
Is (1) bad because the instance [is this word choice correct? Should it be "period"?] between "the box is sent out" and "the letting him know it" may be longer?
(4) Let me know when you send the box.
What about (4)?
Instance is the wrong word - period is better - time would be OK too.
I suspect that there is a "normal" way to say this and that other equally correct sentences sound strange. I also suspect that I am over-analysing these sentences, but that hasn't stopped me before so here we go.
Let's say that you are sending this message to Bill.
Bill is going to send (out) a box to someone, either you or someone else.
You are asking Bill to tell you that he has sent the box and when he sent it as soon as possible after he sends it. You could say:
Please let me know
that you have sent the box and when you sent it as soon as possible after you send it.
But that is much too long-winded and repetitive for normal communications, so you rely on the construction of the sentence and your and Bill's experience of hearing these kind of requests in the past to communicate part of the message - in particular, the part in red.
(1) Please let me know
after you'
ve sent out the box.
(2) Please let me know
after you send out the box.
The use of after in these sentences seems to place some distance between the time of sending out the box and the time of letting me know.
Also, they aren't clear about what it is you want Bill to tell you. Now we all know what you mean, but it could be clearer.
(3) Please let me know
when you'
ve sent out the box.
This time the combination of when and the past perfect form tells Bill that you want the telling to follow the sending very closely.
Also, the use of when says what it is you want to be told, you want to be told when Bill sent the box. When is serving two purposes here - an important point.
(4) Please let me know when you
send the box.
The combination of when with the present form suggests that Bill should tell you something at the same time as he is sending the box. It's a bit like you are asking him to send you some information along with the box.
And finally, I would leave
out out -
sent the box, not
sent out the box