Learn Icelandic: Annar þríhyrningurinn er grænn en hinn er gulur. - One triangle is green but the other is yellow.

In Icelandic, when you want to contrast two things by saying "one is this, the other is that," you use the set phrase annar ... hinn. Even though annar usually means "second" or "another," in this specific pair it translates to "one."

Why does þríhyrningurinn have the definite article "-inn" attached to it?

In the "one... the other..." construction, the noun following annar must have the definite article. If you leave it off and say annar þríhyrningur, the meaning changes from "one of the two triangles" to simply "another triangle" or "a second triangle."

Could I use the word einn (one) at the start of this sentence?

No, because einn is strictly used for counting. If you say einn þríhyrningur, you are emphasizing the number one as opposed to two or three. To express "one of a specific pair," you must use annar.

Why are the adjectives grænn and gulur, as well as the pronouns annar and hinn, in their masculine forms?

They all must agree with the noun they refer to, which is þríhyrningur (triangle). Because þríhyrningur is a masculine noun in the nominative case, all the describing words and pronouns pointing back to it take their masculine nominative endings.