
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.

therefore
Introduces a logical result, consequence, or conclusion from what was said before. It means something like 'deshalb' (therefore) or 'folglich' (consequently).
Du hast lange geübt, nun sollte die Prüfung kein Problem sein.
You've practiced for a long time, so the exam should now be no problem.

well
Used as a filler word or particle to soften a statement, express slight resignation, or continue a narrative.
Nun gut, dann machen wir es eben auf deine Art.
Well then, we'll just do it your way.

well?
Used as a short questioning particle to prompt a reaction or answer. It means something like 'Und jetzt?' (And now?) or 'Was ist?' (What is it?).
Ich habe alles erzählt. Nun? Was sagst du dazu?
I've told you everything. Well? What do you say to that?
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?
then
When the next step follows from an established situation, both dann and nun can work. Dann emphasizes the consequence more strongly, whereas nun highlights the current stage that has been reached or the transition to this step. Dann is suitable for a straightforward consequence, while nun sounds more natural for the current transition.

Die Koffer sind gepackt, nun fahren wir zum Bahnhof.
The suitcases are packed, so now we'll go to the train station.
Die Koffer sind gepackt, dann fahren wir zum Bahnhof.
The suitcases are packed, so we'll go to the train station.