Now

Now is an adverb.

1. Adverb of time

Now means:
(1) "at this time", "at the present time".
(2) "very soon".

Ex:
(1) My friend used to be a miner. Now he's retired.
(2) We'd better eat now; it's eight o'clock.

When now refers to a long period of time, as in (1), we can replace it by nowadays.
Ex: In the old days, people used to go out to enjoy themselves. Now(adays) they stay at home and watch TV.

2. Linking adverb

As a linking word in spoken English, now means "I am changing the subject, and returning to something I was thinking about before."
(1) Now where did we put those maps?
(2) Let's see, now. You must be a couple of years older than me.
(3) Now➘, I have one more point to make ...

With this meaning, now is either unstressed, as in (1), or heavily stressed, as in (3).

3. Idioms

3.1. By now

Let's see example below.
Ex: The bus is late. It should be here by now. (= "before now")

3.2. Now (that)

Now (that) is a subordinating conjunction mixing the meaning of time and reason.
Ex:
(1) Let's have a drink, now (that) you're here. (= "because you are now here")
(2) Now (that) I've learned to drive, I will be able to go to work by car.

I hope this article helps. If you have any question, please leave a comment below.