Learn Icelandic: Sjötti hópurinn var stór.

Sjötti (sixth) is describing hópurinn (the group), which is a masculine noun. In Icelandic, ordinal numbers from third onwards behave like weak adjectives, meaning they end in a vowel. For masculine nouns in the nominative case, that vowel ending is -i.

In English we say "the sixth group," so why isn't there a separate word for "the" before sjötti?

In Icelandic, the "the" part is attached to the end of the noun! The ordinal number comes first, but the noun still takes its definite suffix. So sjötti hópurinn literally translates closer to "sixth group-the."

If we are talking about a specific, definite group, why doesn't the adjective stór take a weak ending?

Because stór is a predicate adjective here, meaning it comes after a linking verb (var) to describe the subject. Predicate adjectives always use the strong declension in Icelandic, regardless of whether the subject is definite or indefinite. So we use the basic masculine singular strong form, stór.

What tells us the gender of hópurinn in this sentence?

The base word for group is hópur. That -ur ending is a strong clue that it is a masculine noun. Additionally, the definite suffix -inn is the standard ending for masculine nouns in the nominative singular.