Questions & Answers about Однажды я был в этом городе.
Why do we use был here instead of the lesson's focus word, бывал?
Why does the sentence use в этом городе (Prepositional case) instead of the Accusative case?
The verb был describes a static state of being in a location, not motion toward it. When talking about where you are located, you must use the Prepositional case. If the sentence were about traveling to the city, like 'I went to this city', you would use the Accusative case for the destination (в этот город).
Could однажды be used to mean 'one day' in the future, like 'One day I will visit this city'?
Not usually. однажды typically points to a specific event in the past, much like 'once' or 'one day' when telling a past-tense story. To say 'someday' or 'sometime' in the future, a Russian speaker would instead use когда-нибудь.
How does the word этом remind us of the gender of город?
The demonstrative pronoun этом is the Prepositional case form for both masculine and neuter nouns. Since город (city) ends in a consonant, it is a masculine noun. If the location were a feminine noun, like страна (country), the pronoun would change to match it: в этой стране.