What it means
さぞ expresses the speaker’s strong inference that something is certainly or surely the case, often with an empathetic sense of “I’m sure...” or “It must have been...”. It commonly modifies a clause followed by an inferential ending such as だろう, でしょう, ことだろう, or ことでしょう. The form is more formal and literary than ordinary expressions such as きっと, and it is especially common when imagining another person’s feelings or circumstances.
Usage note
さぞ usually presents the speaker’s inference rather than directly confirmed information. It is often used with ことだろう or ことでしょう, and さぞかし is a closely related, more emphatic variant; do not treat さぞ as a simple interchangeable replacement for every use of きっと.
How to use it
彼はさぞ驚いたことでしょう。
He must have been surprised.
彼はさぞ驚いたことだろう。
He must have been surprised.
Example sentences
長年暮らした町を離れる彼女は、さぞ心細いことだろう。
She must surely feel lonely as she leaves the town where she has lived for many years.
初めて海外で一人暮らしをする息子は、さぞ不安なことだろう。
My son, who will live alone abroad for the first time, must be anxious.
展覧会を何年も準備してきた主催者は、さぞ悔