nun
now
Indicates the current point in time and is a synonym of 'jetzt' (now).
Früher war hier ein Feld, nun steht hier ein Supermarkt.
There used to be a field here; now there is a supermarket here.

now / well
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now
Indicates the current point in time and is a synonym of 'jetzt' (now).
Früher war hier ein Feld, nun steht hier ein Supermarkt.
There used to be a field here; now there is a supermarket here.

When 'nun' comes at the beginning of a sentence, the conjugated verb usually follows directly. This word order (inversion) is typical when 'nun' introduces a temporal or logical sequence.
Nun beginnt die Vorstellung.
Now the performance begins.
To recognize the meaning, pay attention to the sentence structure. If 'nun' introduces a logical consequence, it is like 'deshalb' (therefore). If it marks a point in time, it is like 'jetzt' (now). As a particle in the middle of a sentence, it often expresses an emotion.
Es hat geregnet. Nun sind die Straßen nass.
It rained. Now the streets are wet.
In the meaning of 'jetzt' (now), 'nun' is often used in written language or in a somewhat more formal, narrative style. It can mark a transition in a story or argument.
Nach langer Diskussion kamen sie nun zu einem Ergebnis.
After a long discussion, they now came to a conclusion.
As a particle, 'nun' is part of fixed expressions such as 'nun ja' (well) or 'nun gut' (all right then). These often express slight resignation or acceptance.
Nun ja, da kann man nichts machen.
Well, there's nothing you can do about that.
The single question 'Nun?' (Well?) is a very direct prompt to speak or react. It creates an expectation in the other person.
Ich habe meine Geschichte beendet. Nun? Was meinst du?
I've finished my story. Well? What do you think?
but
'aber' and 'nun' can both help you continue speaking after a pause in conversations. 'aber' often leads back to the actual topic or introduces a contrast. 'nun' sounds more like a calm transition: you continue the story, soften something, or show that you accept the situation. They can be confused because both sometimes do not carry much information of their own. If you mean a clear contrast, use 'aber'. If you only want to keep speaking or introduce something cautiously, 'nun' fits better.

Nun, bei der Arbeit lernen wir jeden Tag etwas Neues.
Well, at work we learn something new every day.
Im Restaurant ist es gemütlich, aber die Bedienung ist heute sehr langsam.
The restaurant is cozy, but the service is very slow today.