Learn Romanian: Expertul cu care colaborez este foarte inteligent. - The expert I collaborate with is very intelligent.

In Romanian, you can never leave a preposition at the end of a sentence. It must always go directly before the relative pronoun it belongs to. Literally, this sentence translates to "The expert with whom I collaborate is very intelligent."

Does the word care change if the expert is a woman or if there are multiple experts?

No. When used with simple prepositions like cu (with), despre (about), or la (at/to), the relative pronoun care is invariable. It stays exactly the same regardless of the noun's gender or number.

Is the cu strictly necessary here? Can I just say "Expertul care colaborez"?

Yes, cu is absolutely necessary. The verb a colabora (to collaborate) needs the preposition cu (with) to connect to the person. Leaving it out would be like saying "The expert who I collaborate" in English.

How would the sentence change if the expert were a woman?

It would become: Experta cu care colaborez este foarte inteligentă. The masculine noun expertul changes to the feminine experta, and the adjective inteligent gets the feminine ending -ă. As expected, the middle part, cu care colaborez, stays exactly the same.