Please (do something).

Writing "please ___" is a very formal way to ask a question. You can use this for very important requests, or when you don't know the person you're writing to. You can also use it when you are upset with someone at work and don't want to be too friendly with them.

For a friendlier, but still very polite, way to make a request, you can write:

If you could get back to me by the end of the day, that would be appreciated.

get back to (someone)

To "get back to" someone means to respond to their question later. For example, if you ask me a question and I don't know the answer, I can say:

I don't know off the top of my head, but I'll get back to you this afternoon.

(do something) by (a certain time)

You use the word "by" to indicate a deadline.

"Before" is similar in meaning to "by", but "before" doesn't carry the same meaning of "deadline". Instead, it's used when you need to do things in a certain order:

Remember to get Henry's approval before you present it to the client.

This is something you would say if a coworker is going to make a proposal to a client, but she is supposed to ask Henry, who's the Director, for permission first.

You would use "by" in this way:

We're presenting this on Monday morning, so remember to get Henry's approval by Friday afternoon.

the end of the day

When you say "the end of the day" in a work situation, it means 5:00, 6:00, or whatever time most people leave work.