erstmalig
for the first time
Describes that something happens, takes place, or is done for the very first time.
Das Festival findet dieses Jahr erstmalig in unserer Stadt statt.
The festival is taking place in our city for the first time this year.

for the first time
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for the first time
Describes that something happens, takes place, or is done for the very first time.
Das Festival findet dieses Jahr erstmalig in unserer Stadt statt.
The festival is taking place in our city for the first time this year.

The word 'erstmalig' (for the first time) is a more formal and expanded form of 'erstmals' (for the first time). It is derived from the combination 'zum ersten Mal' (for the first time) and becomes an adjective or adverb through the ending '-ig'.
Die erstmalige Durchführung des Projekts war ein großer Erfolg.
The first-time implementation of the project was a great success.
'Erstmalig' (for the first time) can be used as an adverb to describe a verb. But it can also stand before a noun as an adjective, and is then declined like any other adjective.
Die Konferenz wurde erstmalig online abgehalten.
The conference was held online for the first time.
'Erstmalig' (for the first time) belongs more to formal language and is often used in official announcements, reports, or the press. In everyday language, people usually say 'zum ersten Mal' (for the first time).
Der Preis wurde in diesem Jahr erstmalig an eine Frau verliehen.
The prize was awarded to a woman for the first time this year.
only / not until / first
'erst' and 'erstmalig' look similar, but they do not mean the same thing. You use 'erst' when something does not happen before a certain point in time, when something is only a small amount, or when you are describing a sequence. 'erstmalig' always means that something is happening for the very first time. The confusion arises because both can have to do with a beginning or an early time. If you mean 'for the first time', use 'erstmalig'. If you mean 'not earlier than', 'only', or 'first', then 'erst' fits.

In diesem Jahr findet der Kurs erstmalig online statt.
This year, the course is being held online for the first time.
Der Supermarkt öffnet heute erst um zehn Uhr.
The supermarket doesn't open until ten o'clock today.