Express your gratitude（感謝します）

My immediate supervisor has decided to quit his job. I was very surprised at the suddenness of the situation. I am very sorrowful because he has been a great help to me since I joined the company. Also, I will be taking over his work, which means I will have more work to do. I would like to express my gratitude to him before he leaves.

immediate　直接の、即時の、早速の
suddenness　突然
gratitude　感謝
which means　つまり、言い換えれば

直属の上司が仕事を辞めることになりました。突然のことに大変驚きました。入社当時から大変お世話になっていたので、とても悲しいです。また、仕事を引き継ぐのでこれからさらに仕事が増えそうです。退職されるまえに今までの感謝を伝えたいと思います。