What is the difference between them? I see many 'nunmehr' when it seems to me that 'nun' is also okay.
Typically nun refers to a moment in time, which can be both in the past or in the future. For example: Was ist nun geschehen? Was machen wir nun?
Nunmehr on the other hand refers to a period of time, which can also be in the past (specifying a number for the amount of time) or in the future (open ended). For example: Seit nunmehr zwei Wochen regnet es. Wir wollen uns nunmehr vertragen.
But I agree there is some overlap and in some cases nun might be used in place of nunmehr (but not the other way around).
There are some other uses of nun:
- to start a question: Nun, was ist Ihre Entscheidung?
- to continue an item in a list: Nun kommen wir zum zweiten Punkt auf der Tagesordnung.
- as a fill word to gain time when thinking about something: Nun... du hast Recht.
The use of nunmehr is quite rare and a bit oldfashioned, but can be a fine tuning- a means to stress something. You can use nun nearly always instead of nunmehr (without that emphasis). In the following there is only one sentence, you cannot.
In many cases, nunmehr could be let away completely without that the sentence would get grammatically incorrect or a changed meaning, because it is only an emphasis- it stresses something.
The two most common meanings are:
First- usage- in a context to a past time period, which shall be stressed:
Because of that, mostly an explicit period of time is mentioned as with since or for:
(for [more than] 30 years)
Seit [nunmehr] [über] 30 Jahren sind wir verheiratet.
(for the past 12 years)
Seit [nunmehr] 12 Jahren...
[Nunmehr] seit 1992 existiert unsere Firma.
(only sentence here where it can´t be replaced by nun.)
Second- usage as a future lookout: A merely infinite prospect/lookout in the future where it can have a meaning of from now on as well as meanwhile (Infinite in the sense that it is the opposite to a time period.)
Nunmehr bleibt ihm keine Alternative.
Nunmehr kostet das Ticket 6 EUR.
Dictionaries seem to mention more meanings. In other contexts than mentioned I would judge it's use questionable in modern German, for example, if it can be used as a general substitution of jetzt without context to the past (1.) or the future (2.) .
Wir können [nunmehr] aufatmen.
(Correct and does not reference a time period, but has in fact implicit associations both to (fear in the) past and future relief.)
Kommen wir [nunmehr/nun] zum zweiten Punkt der Tagesordnung.
(There is probably a small emphasis, hint or a subtext here, when using nunmehr. For example, the first topic took too long, the speaker wants simply to be very formal, or to give a hint to speed up a bit, maybe accompanied with facial expressions.)