Learn Spanish (Spain): Los directivos con quienes colaboré eran muy estrictos. - The executives with whom I collaborated were very strict.

Quienes is the plural form of the relative pronoun quien (who/whom). We use it here because it refers back to the plural noun Los directivos (the executives). Since it follows the preposition con (with) and refers specifically to people, con quienes (with whom) is the correct choice.

Could I use que instead of quienes in this sentence?

Yes, you could! After short, simple prepositions like con, de, or en, you can use que instead of quien or quienes, even when referring to people. So, Los directivos con que colaboré is also grammatically correct, though con quienes sounds a bit more natural and specific when talking about people.

Why does this sentence use both the preterite (colaboré) and the imperfect (eran) past tenses?

This is a classic example of mixing past tenses to set a scene. Colaboré (I collaborated) is in the preterite because your work with them was a completed action or specific project. On the other hand, eran (they were) is in the imperfect because it describes an ongoing characteristic or background detail—being strict was just how they were during that time.

Does directivos mean exactly the same thing as "directors"?