Learn Spanish (Latin America): Conocimos al pionero cuyo proyecto había cambiado la ciudad. - We met the pioneer whose project had changed the city.

The word cuyo translates to "whose". It acts like an adjective, so it must match the gender and number of the thing being owned, not the owner. Since it describes el proyecto (masculine singular), we use cuyo, even though it refers back to el pionero. If the pioneer's idea had changed the city, it would be cuya idea.

Why does conocimos mean "we met" instead of "we knew"?

The verb conocer generally means "to know" a person or place. However, when you use it in the preterite tense for completed actions (like conocimos), it specifically means meeting someone for the first time. To say "we knew the pioneer" over an ongoing period of time, you would use the imperfect tense: conocíamos.

Why do we say al pionero instead of just el pionero?

This is the "personal a" combined with the word el. In Spanish, when the direct object of a verb is a specific person, you must put an a in front of it. Here, the preposition a and the article el combine to make the single word al.

What verb tense is había cambiado?