He is an expert in analytical mechanics and deeply understands its theory.

The lens of this camera is easy to attach and remove.

He is learning Greek.

He went on a trip with his Greek friend.

We are planning to set off early tomorrow morning.

My departure is at 8 a.m.

He read that book three times.

Please open the door.

Every morning, I open the window to let in fresh air.

This area is still undeveloped.