さぞ(かし)
surely, must have been (imagining someone's feelings)
さぞ(かし) + 〜でしょう / 〜ことだろう / 〜に違いない
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さぞ imagines how strongly someone must feel, especially in a situation you weren't in yourself: 長旅でさぞお疲れでしょう, "you must be exhausted after such a long journey".
It almost always ends with a guess: でしょう, だろう, ことだろう or に違いない. The feeling is usually strong, like tiredness, joy, sadness or anger, and さぞ adds sympathy.
さぞかし and さぞや are more emphatic versions.
It's close to きっと, but きっと is just a confident guess about anything. さぞ is specifically "I can only imagine how…". That makes it polite and warm, and common in letters, condolences and congratulations.
You don't use it about your own feelings.
Examples
長旅でさぞお疲れでしょう。
You must be exhausted after such a long journey.
合格して、ご両親もさぞお喜びでしょう。
Your parents must be delighted that you passed.
一人でさぞ寂しかったことだろう。
You must have been so lonely on your own.
山頂からの景色はさぞ美しかったに違いない。
The view from the summit must have been beautiful.
あんなことを言われて、さぞ腹が立ったでしょう。
You must have been furious to be told that.