selten
rare
Not occurring or existing often; scarce.
Diese Briefmarke ist extrem selten und wertvoll.
This stamp is extremely rare and valuable.

rare / seldom
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rare
Not occurring or existing often; scarce.
Diese Briefmarke ist extrem selten und wertvoll.
This stamp is extremely rare and valuable.

'Selten' can describe a noun as an adjective or describe how often something happens as an adverb. As an adjective, it is declined; as an adverb, it remains unchanged.
Das ist eine seltene Pflanze.
That is a rare plant.
As an adverb of frequency, 'selten' (seldom) usually comes directly after the conjugated verb and before other parts of the sentence.
Ich esse selten Fleisch.
I seldom eat meat.
In the meaning 'rar' (rare), 'selten' is often combined with nouns such as 'Gelegenheit' (opportunity), 'Gast' (guest), 'Exemplar' (specimen / copy), or 'Anblick' (sight).
Ein solches Angebot ist eine seltene Gelegenheit.
Such an offer is a rare opportunity.
never
'nie' and 'selten' can both express that something is not normal or does not happen often. But the difference is: 'nie' means not at all, so zero times. 'selten' only means very little or at long intervals. Learners can easily confuse the words because both feel like negative frequency words. If something maybe happens once a year, it is 'selten'. If it does not happen at all, it is 'nie'.

Heute koche ich seltene Gewürze nicht, sondern nehme einfach Salz und Pfeffer.
Today I'm not using rare spices; I'm just using salt and pepper.
Im Urlaub schreibt ihr nie E-Mails für das Projekt.
On vacation, you never write emails for the project.