Learn Russian: Гагарин — первый человек, который увидел нашу планету из космоса. - Gagarin is the first person who saw our planet from space.

Surnames ending in -ов, -ев, and -ин like Гагарин decline mostly like regular masculine nouns, but they have one major exception: in the Instrumental case, they take the adjective ending -ым instead of -ом. So "with Gagarin" translates to с Гагариным. The Prepositional case remains regular, so "about Gagarin" is о Гагарине.

Why is there a dash (—) right after Гагарин?

In Russian, the present tense of the verb "to be" is skipped. When you link two nouns in the Nominative case (like the subject Гагарин and the noun it equals, человек), you use a dash in writing to represent that missing "is".

Why is который used to mean "who" here instead of кто?

While кто is used as a question word or in general statements (like "everyone who"), который is a relative pronoun that links a descriptive clause to a specific noun. Because "who" refers directly back to the specific noun человек (person), you must use который. It acts like an adjective and takes the masculine singular form to match.

Why does the sentence use the perfective verb увидел instead of the imperfective видел?