How and when should I use "By now"? [closed]

What does "by now" stand for?

Where should I put it within a sentence? starting or finishing? Is it right if I say "By now, I won't go to Melbourne"?

To understand the phrase by now, I suggest starting with understanding the sense of by that is in use. In this phrase, by indicates the end of a particular period of time.

I have to finish writing this answer by midnight.

I expect the package to arrive by tomorrow.

Of course now simply means the present moment, so by now means "a period of time ending at the present moment."

So no, it is not right to say By now, I won't go to Melbourne and I am not sure what you mean by that.

You can put by now at the beginning or at the end of a sentence. It's a matter of what you want to emphasize. If we are on a train and it is now past the scheduled arrival time of the train in Melbourne, I could say:

We should be in Melbourne by now. (emphasizes where we should be)

By now we should be in Melbourne. (emphasizes how much time has passed)

I might say the first sentence if I know what time it is but do not know where we are (as is generally the case when I'm on a train). The second is more of a complaint about how late the train is running when I know the schedule and I know we are a long way from Melbourne (and probably my destination is even farther away).

But these are not hard and fast distinctions. If I were upset about the train running late, I might use the first sentence to express my displeasure to the train conductor, even though I know where we are (different sense of should).