He is proud to be a subject of the emperor.

The astronomers began observing the nova.

This experiment is conducted in a vacuum.

I went to the department store to buy new bedding.

He is majoring in psychology.

The city becomes quiet when it's midnight.

It's difficult to understand his mental state.

The doctor checked my electrocardiogram.

It's difficult to understand his frame of mind.

His curiosity always drives him to learn new things.