我当下就决定了。
wǒ dāngxià jiù juédìng le 。
I decided right away.
当下我感到非常开心。
dāngxià wǒ gǎndào fēicháng kāixīn 。
I felt very happy at that moment.
他当下没有时间去想这个问题。
tā dāngxià méiyǒu shíjiān qù xiǎng zhègè wèntí 。
He didn't have time to think about this issue at that moment.