Путешественник сидит у костра.

to sit

Breakdown of Путешественник сидит у костра.

• у: at
• костёр: the campfire
• путешественник: the traveler

Questions & Answers about Путешественник сидит у костра.

Why is it костра and not костёр in this sentence?

Because of the preposition у.

• The basic noun is костёр (campfire), nominative singular.
• у always requires the genitive case after it.
• The genitive singular of костёр is костра.

So:

• костёр = campfire (subject form, dictionary form)
• у костра = by/at the campfire (literally: at [of] the campfire)

This is the same pattern you will see with many other nouns after у:

• у стола (by the table) ← from стол
• у окна (by the window) ← from окно

What exactly does у mean in у костра, and why does it use the genitive?