constant / continuous
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continuous
Something that continues or happens without interruption over a longer period of time; uninterrupted.
Der andauernde Lärm von der Baustelle macht mich wahnsinnig.
The continuous noise from the construction site is driving me crazy.
'Andauernd' (continuous/constant) is the present participle of the separable verb 'andauern' (to continue/last). The verb means 'fortbestehen' (to continue to exist) or 'weitergehen' (to go on). The prefix 'an' here emphasizes the beginning and continuation of a state.
Die Kälteperiode dauert schon seit Wochen an.
The cold spell has been continuing for weeks.
The word can stand as an adjective before a noun or as an adverb that describes a verb. As an adjective it is declined; as an adverb it remains unchanged.
Der andauernde Regen ist schlecht für die Ernte.
The continuous rain is bad for the harvest.
In the meaning 'ständig wiederholt' (constantly repeated), 'andauernd' is mainly used colloquially. It often expresses that the speaker is annoyed by the repetition.
Mein Handy klingelt andauernd, ich kann mich nicht konzentrieren.
My phone keeps ringing constantly; I can't concentrate.
The neutral meaning describes an uninterrupted state (e.g., pain, noise). The colloquial, negative meaning describes a constantly repeated action (e.g., calling, asking).
Er leidet unter andauernden Kopfschmerzen.
He suffers from continuous headaches.
long
'lang' and 'andauernd' can both express that something involves a lot of time. 'lang' mainly describes duration: 'ein langer Film' (a long film), 'eine lange Wartezeit' (a long wait). 'andauernd' emphasizes more strongly that something continues, does not stop, or happens again and again. In colloquial language, 'andauernd' often sounds annoyed, for example with noise or interruptions. If you only mean the length of time, 'lang' fits better. If you want to say that something keeps going or annoys you, 'andauernd' is more natural.
Im Büro klingelt andauernd das Telefon.
The phone in the office keeps ringing.
Die Sitzung war heute Morgen länger als geplant.
The meeting was longer than planned this morning.