さぞ

さぞ means "surely", "how ... you must be": you imagine what someone else is feeling or going through and say you can picture it. The sentence ends in a guess – だろう, でしょう, ことだろう or にちがいない違いない.

Without the guess at the end, the sentence doesn't work: さぞ is imagining, not stating.

• × さぞ大変でした → さぞ大変だったでしょう

A weak guess like かもしれない doesn't fit either: さぞ means you are sure.

It is about someone or something else – a person's feelings, a place you picture – never about your own feelings, which you know rather than imagine.

ことだろう and さぞかし

ことだろう and ことでしょう make the sentence more emotional and more literary. さぞかし is a stronger form of さぞ and goes in the same places.

Register

さぞ is formal and a little literary. It suits condolences, letters and polite conversation with someone older. Among friends, きっと with でしょう or だろう, or just the guess on its own, is more natural.