Questions & Answers about Третий урок интересный.
All three words are masculine nominative singular.
- урок (lesson) is the subject of the sentence and takes the nominative.
- Both третий (ordinal number) and интересный (adjective) agree with урок in gender, number and case.
Ordinal numbers in Russian behave like adjectives: they have endings for gender, number and case. From the base cardinal number три:
• Masculine Nom. Sing. → третий
• Feminine Nom. Sing. → третья
• Neuter Nom. Sing. → третье
• Plural Nom. → третьи
And each of these will take the appropriate endings in other cases (e.g. третьего, третьему etc.).