Breakdown of Мы часто вспоминаем Гагарина, который был первым в космосе.
мы
we
часто
often
в
in / to (direction)
вспоминать
to remember, to recall
Questions & Answers about Мы часто вспоминаем Гагарина, который был первым в космосе.
Why does the surname Гагарин become Гагарина here?
The verb вспоминать (to remember) takes a direct object in the accusative case. Since Gagarin is an animate person, the accusative matches the genitive case. For masculine surnames ending in -ин, the genitive/accusative ending is -а, just like a regular masculine noun. The special surname rules you are learning mostly differ from standard nouns in the instrumental case, which would be Гагариным.
Why do we use который instead of кто for 'who'?
When 'who' introduces a descriptive clause linking back to a specific noun in the main sentence (here, Gagarin), you must use the relative pronoun который. You use кто for actual questions or for generic statements like 'Those who...'.
If который refers back to Гагарина (which is accusative), why is который in the nominative case?
A relative pronoun gets its gender and number from the noun it refers to (masculine singular for Gagarin), but its case comes from its job in its own clause. In the phrase который был первым, the word 'who' is the subject of the clause, so it must be in the nominative case.
Why is первым in the instrumental case instead of the nominative первый?