One and the other (einn... annar)

To contrast options from a larger group, use einn ... annar (one ... another). These pronouns agree in gender and case with the noun they describe.

For a masculine noun, use einn ... annar:

One advantage is the price, but another is this disadvantage right here.
Einn kostur er verðið, en annar er þessi galli hérna.

For a feminine noun, use ein ... önnur:

One version has this characteristic, but another does not have it.
Ein útgáfan hefur þetta einkenni, en önnur hefur það ekki.

Specific Pairs: "One and the Other"

To contrast exactly two specific things, use annar ... hinn (one ... the other). The first item takes a form of annar and the second takes hinn. Because these refer to specific things, the noun usually takes the definite article.

For a masculine noun, use annar ... hinn:

One disadvantage is the appearance, but the other is the price.
Annar gallinn er útlitið, en hinn er verðið.
One part is old, but the other is from the original.
Annar parturinn er gamall, en hinn er úr frumritinu.

For a feminine noun, use önnur ... hin:

The difference is that one side is black but the other is white.
Munurinn er sá að önnur hliðin er svört en hin er hvít.

For a neuter noun, use annað ... hitt:

Here are two documents, one is the original but the other is the copy.
Hér eru tvö skjöl, annað er frumritið en hitt er afritið.