Learn German: Das moderne Dach war sehr teuer. - The modern roof was very expensive.

Because it comes directly after the definite article das. In the nominative case (when the noun is the subject of the sentence), adjectives for singular nouns following a definite article (der, die, or das) always get an -e ending.

Why doesn't teuer get an ending like moderne does?

Adjectives in German only get endings when they are placed directly in front of the noun they describe. Because teuer comes after the verb war, it is a predicate adjective and stays in its basic dictionary form without any extra letters.

Is war the past tense form of ist?

Yes! The word war is the simple past tense of sein (to be). Since Das moderne Dach is a third-person singular subject, we use war, which translates exactly to the English word was.

How do we know the gender of Dach in this sentence?

The definite article das right at the beginning tells us that Dach (roof) is a neuter noun. Knowing this is what allows you to choose the correct adjective ending for moderne!