frühmorgens
early morning
At a very early time in the morning.
Frühmorgens um fünf Uhr ist die Stadt noch ganz still.
Early in the morning at five o'clock, the city is still completely quiet.

early in the morning
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early morning
At a very early time in the morning.
Frühmorgens um fünf Uhr ist die Stadt noch ganz still.
Early in the morning at five o'clock, the city is still completely quiet.

The word is made up of 'früh' (early) and 'morgens' (in the morning). It emphasizes that something happens not just in the morning, but at a particularly early hour.
Der Bäcker beginnt seine Arbeit frühmorgens.
The baker starts his work early in the morning.
As an adverb, 'frühmorgens' (early in the morning) can stand at the beginning of the sentence (position 1), which moves the verb to position 2. But it can also stand after the verb in the middle of the sentence.
Frühmorgens trinke ich immer einen Kaffee.
Early in the morning, I always drink a coffee.
'Frühmorgens' (early in the morning) is often combined with verbs that describe the start of the day, such as 'aufstehen' (to get up), 'losfahren' (to set off), or 'aufwachen' (to wake up).
Wir müssen frühmorgens losfahren, um den Stau zu vermeiden.
We have to set off early in the morning to avoid the traffic jam.
early
'früh' and 'frühmorgens' can both describe a time at the beginning of the day. 'früh' is more general: it can be in the morning, but it can also simply mean earlier than expected. 'frühmorgens' is more specific and means a very early time in the morning. It fits when you want to clearly name the part of the day, for example before work or before many normal appointments. So 'früh' can refer to many early times; 'frühmorgens' refers specifically to very early in the morning.

Frühmorgens ist er im Büro oft allein und kann ruhig arbeiten.
Early in the morning, he is often alone in the office and can work in peace.
Die Klasse musste früh los, damit der Ausflug pünktlich beginnen konnte.
The class had to leave early so that the excursion could start on time.