Questions & Answers about Вчера мы долго гуляли по городу.
Why do we use по городу instead of в городе? I thought they were walking in the city.
The preposition по followed by the Dative case is used for movement around, along, or all over a surface or area. While в городе (Prepositional case) just tells you the static location where the walking happened, по городу specifically paints the picture of wandering around the city streets.
Why does город get an -у ending here?
The preposition по forces the noun that follows it to take the Dative case. Because город (city) is a masculine noun ending in a consonant, its Dative ending is -у, making it городу.
Why do we use the imperfective verb гуляли here instead of a perfective verb?
The adverb долго (for a long time) is a big clue! Whenever you focus on the duration or process of an action rather than its completion, you need the imperfective aspect. Plus, гулять (to stroll or walk for pleasure) is almost always imperfective because strolling is naturally a continuous process.