...That...

That

I saw the man, who was with Sarah.

من مردی را دیدم که با سارا بود./man mardi ra didam ke ba sara boud

I found the notebook, which I've lost.

من دفتری را پیدا کردم که گم کرده بودم./man daftari ra peida kardam ke gom kardeh boudam

If you want to use "that" in a Farsi sentence, you can always ad /ke/ in the right position. Don't care if it is who, whom, which, that or anything else.