This advice article includes a list of phrases for a condolence letter that are respectful, sympathetic, and kind.
"I was so sorry to hear about the loss of _____________."
"I was deeply saddened to hear about ________________'s death."
"My deepest sympathies go out to you."
"I'm praying for you."
"My thoughts are with you during this difficult time."
"Please accept my condolences on the passing of your ____________."
"I'd like to offer my sincerest sympathies concerning the death of _____________."
"Words cannot express how sorry I am for your loss."
"Although I can't understand the depth of your loss�"
"My heart goes out to you and your family."
"I was shocked and saddened to hear about the loss of _________________."