solemnly

厳粛に誓う

To make a formal and serious vow, often in a legal context.

公式で威厳のある方法で真剣な約束をする

厳粛に約束する

To make a serious and formal promise.

真剣で正式な約束をする。

To state something formally and with great seriousness.

厳粛に言う

To speak words in a grave and serious tone.

重々しく真剣な口調で言葉を話す。

厳粛にうなずく

To move one's head down and up as a sign of agreement or understanding in a serious context.

真剣な文脈で、同意や理解のしるしとして頭を上下に動かす。