Learn Danish: Det er den mest spændende udstilling i byen. - It is the most exciting exhibition in the city.

The word spændende (exciting) is a present participle, which you can spot by the -ende ending. Words ending in -ende or -et are considered too long to take the usual -ere and -est endings. Instead, we use mere (more) and mest (most) before the word, just like in English.

Why does it say den mest spændende udstilling instead of mest spændende udstillingen?

When you put an adjective or a descriptive phrase in front of a noun in Danish, you drop the noun's attached definite ending (like the -en in udstillingen). Instead, you use the freestanding definite article den in front of the whole phrase.

Since we are using the definite article den, shouldn't the adjective add an -e at the end?

Normally, yes! Regular adjectives get an -e ending when used with a definite article like den. However, adjectives that already end in -ende are exceptions. They are completely locked and never change their endings for gender, plural, or definiteness.

Could I use på byen instead of i byen?

Not in this sentence. The phrase i byen means something is physically located inside the city limits. If you say på byen, it is a special idiom that means going out to party or drink, similar to saying out on the town in English.