He was dumfounded by her unexpected response.

I was dumfounded when I realized I had been pronouncing his name wrong all this time.

She dumfounds me with her intelligence.

Her performance was dumfounding.

Her unexpected response dumfounded me.

Her unexpected response left me dumfoundering.

The magician dumfounders the audience with his incredible tricks.

I was completely dumfoundered by his unexpected announcement.

Her unexpected talent for singing dumfounded the audience.