
momentan
currently
At the present time; currently or at the moment.
Momentan habe ich leider keine Zeit für ein Treffen.
Unfortunately, I currently don't have time for a meeting.
currently
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currently
At the present time; currently or at the moment.
Momentan habe ich leider keine Zeit für ein Treffen.
Unfortunately, I currently don't have time for a meeting.

instantaneous
In physics and technology: related to a specific moment or instant; instantaneous.
Der Tacho zeigt die momentane Geschwindigkeit des Fahrzeugs an.
The speedometer shows the vehicle's instantaneous speed.
The adverb 'momentan' (currently) is derived directly from the noun 'der Moment' (the moment). So it describes a state that is valid for the present moment.
Einen Moment bitte, ich bin gleich für Sie da.
One moment please, I'll be right with you.
'Momentan' means 'zurzeit' (at the moment) or 'gegenwärtig' (currently). Don't confuse it with the English 'momentarily', which often means 'in Kürze' (in a moment).
Die Chefin ist momentan in einer Besprechung.
The boss is currently in a meeting.
As an adverb, 'momentan' (currently) is flexible. It can stand at the beginning of the sentence in position 1 (then the verb follows), or in the middle field of the sentence, usually after the verb and the subject.
Momentan ist das Restaurant geschlossen.
The restaurant is currently closed.
You can recognize the physical-technical meaning from the context. It appears almost exclusively in technical language, often in connection with measured values such as 'Geschwindigkeit' (speed), 'Leistung' (power), or 'Beschleunigung' (acceleration).
Der Sensor misst den momentanen Druck im System.
The sensor measures the instantaneous pressure in the system.
now
Jetzt refers to the present and often highlights a change from before. Momentan describes a current state that may appear temporary. Both can be used for present states; the intended perspective, whether on change or limited duration, determines the choice.

Wegen einer Reparatur fährt Nina momentan mit dem Fahrrad.
Nina is currently riding her bike because of repairs.
Früher fuhr Nina mit dem Bus, aber jetzt fährt sie mit dem Fahrrad.
Nina used to take the bus, but now she rides her bike.