Learn Icelandic: Fyrsti bíllinn var rauður, en annar bíllinn var blár. - The first car was red, but the second car was blue.

Ordinal numbers in Icelandic (except for annar, which means second) generally act like weak adjectives. This means they end in a vowel in the nominative case. So for a masculine noun like bíll, you use fyrsti (first) instead of the strong ending -ur.

If ordinals usually end in a vowel, why is the word for second annar instead of having an -i ending?

Annar is the big exception to the rule! It behaves like a strong adjective and changes its ending depending on the gender of the noun. For a masculine noun, it is annar, for feminine it is önnur, and for neuter it is annað. It can also mean other or another.

Why does bíllinn have the definite "the" ending attached when there is an ordinal number in front of it?

In English, we say the first car. In Icelandic, you don't usually use a separate word for the. Instead, the ordinal number just sits directly in front of the noun with its attached the ending, literally translating to first car-the (fyrsti bíllinn).

The noun bíllinn is definite, so why do the colors rauður and blár use the strong -ur and -r endings?

Because they come after the verb var (was). When an adjective describes a noun after a to be verb, it is called a predicate adjective. Predicate adjectives always use their strong forms, no matter if the noun they describe has a definite ending or not.